Shoggoth.net Corner Brood Shoggoth.net

Incident dossier B-5: colonial life at geometric discontinuities

The landlord blamed subsidence.

The first crack showed up above the pantry door. The second branched off it at exactly ninety degrees. By morning there were seven, each running to meet another surface without ever crossing it. The tenants heard scratching, though microphones pressed flat to the walls picked up nothing at all.

On the third day a surveyor fed a fibre-optic camera into the plaster. The void behind the wall held no insects, no nest, no moisture, nothing moving. It held blue dust, arranged in a single line along the seam where two surfaces met. While the surveyor watched the monitor, the line grew another angle. The camera cable came back in two pieces, and the cut was perfectly flat.

The name is wrong

People keep calling Corner Brood juvenile Hounds of Tindalos, because they are small, they turn up after Hound manifestations, and they multiply fast. Nobody has ever watched one grow into a Hound. They might be parasites carried across on a Hound’s projected body, or scavengers drawn to the energetic wreckage a manifestation leaves behind, or completely unrelated angular organisms taking advantage of the same breach. Whether the connection is biological, ecological, or just geometric, we do not know.

Even brood oversells it. No egg has ever been recovered. Nothing has been seen dividing. An infestation looks like it reproduces because every occupied corner spawns fractures, and every fracture is one more place the colony can live. Their projected bodies are the corners.

A real place for an impossible animal

A ninety-degree wall junction is not a tear in spacetime and it is not a portal. What makes it matter is the ordinary, well-documented ability of boundary geometry to trap physical modes.

Take a field in a two-dimensional wedge of opening angle $\theta$ , which is just a cross-section through an edge, and put an attractive Robin boundary condition on both walls:

$\partial_\nu\psi=\alpha\psi,\qquad \alpha>0$

Robin conditions tie a field’s value at a boundary to its normal derivative, and they turn up in wave mechanics, acoustics, heat transfer, diffusion, and any quantum system with a surface. No occult physics required. For an acute or convex wedge, the function

$\psi_\theta(r,\phi)=C\exp\!\left[-\frac{\alpha r\cos\phi}{\sin(\theta/2)}\right]$

sits localized at the vertex, with eigenvalue

$\lambda_\theta=-\alpha^2\csc^2(\theta/2)$

A flat boundary only reaches down to $-\alpha^2$ , so the wedge throws in a binding advantage,

$B_\theta=\alpha^2\cot^2(\theta/2)$

over a localization length

$\ell_\theta=\frac{\sin(\theta/2)}{\alpha}$

The reading is simple. Open the wedge toward a flat plane and the advantage evaporates. Sharpen it and the mode is gripped harder and squeezed into a smaller region, which is the uncomfortable part: a hairline crack can hold the thing more tightly than the corner of a room. In three dimensions the hierarchy is even blunter. A surface gives $\psi=e^{-\alpha x}$ with $\lambda=-\alpha^2$ . A right-angle edge gives $\psi=e^{-\alpha(x+y)}$ with $\lambda=-2\alpha^2$ . A three-wall vertex gives $\psi=e^{-\alpha(x+y+z)}$ with $\lambda=-3\alpha^2$ . Every independent intersecting boundary binds harder.

That is all legitimate spectral geometry. The fiction adds one thing: Corner Brood are angular organisms whose projected biology rides on these boundary-localized modes. Take away the mode and there is no ordinary-space body underneath. The habitat and the animal are the same physical excitation, which is why nobody has ever dissected one.

What an infestation looks like

Corner Brood have no stable appearance away from the defect holding them up. Look directly and you get different signs depending on who is looking and with what:

Blue-black particulate that migrates against gravity toward vertices.
A wet sheen confined to the line where two surfaces meet.
Tiny jointed shadows with no objects to cast them.
Hair-thin filaments visible only when your eye focuses past the corner.
Rows of dark pits that vanish when you approach them head-on.
A sound like claws moving around inside your own teeth.

Microscopy rarely helps. Material lifted from an occupied corner goes inert the moment its geometry is disrupted, and under magnification it looks like vitrified protein, or metallic oxide, or damaged photographic emulsion, depending entirely on how you prepared it. Try to cut a stable sample holder and you have just made smaller corners. The most reliable test is behavioural: loose particles drift toward the sharpest available defect and stop. Round that defect off and they migrate toward the cracks under the rounding rather than dispersing. And the apparent organisms strung along the junction are not separate animals in any familiar sense. They are high-amplitude knots in one colonial mode running through the connected defect network. Counting them is like counting waves and pretending there is no sea.

How it spreads

A colony cannot cross an open, smooth floor the way an animal does. It spreads three ways.

Fracture propagation

The colony loads the tip of its occupied crack until the crack lengthens, branches, or meets another boundary, and every new junction is another bound state. In plaster or dry timber you might hear it. In glass, masonry, bone, or tooth enamel you might not notice until something fails. The branching angles are unnaturally exact, repeating at 30, 45, 60, and 90 degrees even where grain, load, and material should force a ragged break.

Mode tunnelling

Two similar corners a distance $L$ apart carry overlapping localized modes whose interaction falls off roughly as

$\Delta\lambda\sim A e^{-L/\ell_\theta}$

with $A$ set by the geometry and the boundary material. When the gap is small compared to the localization length, the colony can jump between defects without touching the surface in between. To a witness, one corner goes quiet as the far corner starts scratching, and nothing has crossed the room.

Transport while dormant

Blue residue is not itself an active organism, but it can hold onto the coupling needed to seed one. Contaminated plaster dust, drill bits, filters, clothing, evidence bags, and vacuum cleaners carry inert-looking material into fresh geometry, and once it is pressed into a favourable defect the projected mode reforms. Clean an infested room with an ordinary vacuum and you may have just seeded every corner the machine and its dust bag pass through for the rest of the week.

Why it attacks living tissue

Curved biological tissue is poor habitat, so the Brood renovate it. Skin folds, joints bend, teeth meet, and bone is full of branching interfaces and microscopic cracks. All of it is inferior to an architectural vertex but close enough for a dense colony to work with. The Brood concentrates stress until a smooth or rounded structure develops a discontinuity that binds it more tightly. The result is faceting. A crease goes too straight. A knuckle locks at an exact angle. A molar splits along a flawless plane. Scar tissue grows polygonal edges. Under heavy infestation a long bone does not shatter at random; it divides into geometric solids whose faces are disturbingly clean. Faceting turns the victim into more habitat.

Brood Density

Track each physically connected infestation with Brood Density, 0 to 6. It measures the number, quality, and connectivity of the occupied defects, not a head count.

Density	State	Observable effect
0: Absent	No viable mode remains.	Residue is inert unless introduced to a new favourable defect.
1: Seeded	One weakly occupied corner or crack.	Migrating dust, cool surfaces, faint scratching. No direct attacks.
2: Cluster	Several nearby defects support a connected mode.	Cracks lengthen; tools and cables suffer planar cuts.
3: Room	The colony holds a working network within one space.	Mode tunnelling, Edge Cuts, and early Faceting become possible.
4: Structure	Walls, floors, ducts, framing, or utilities link multiple rooms.	It controls routes through the building and spreads fast.
5: Bloom	New fractures form faster than routine repair can close them.	Living occupants become preferred sites for new geometry.
6: Tessellation	The structure is turning into a self-similar angular habitat.	Collapse, mass Faceting, and contamination of neighbouring buildings are imminent.

Raise Density when an occupied crack reaches a new boundary or branches into several viable tips, when construction or drilling or demolition opens many untreated defects, when contaminated material gets pushed through ventilation or cleaning, when a Hound manifests and sheds active residue into the colony, when separate local clusters become geometrically connected, or when a botched repair seals the visible surface and leaves sharper voids underneath. Lower it when occupied defects are fully injected and rounded past the colony’s localization scale, when contaminated material is kept away from all rigid intersections, when parts of the network are cut apart without generating viable debris, when tuned vibration or thermal cycling disrupts the mode long enough to physically remediate, or when every active defect is removed or turned into an unfavourable boundary. Do not touch Density for cosmetic cleaning, painting, a skim of plaster, or killing an affected person. Those remove signs, not habitat.

Cthulhu Eternal statistics

A colony is an environmental organism with no central body, and you cannot beat it by reducing some creature to 0 HP. It is also one entity per infestation rather than a species you roll up, so the profile below is fixed; it describes the colonial mode as it acts through occupied geometry.

CORNER BROOD COLONY, life in the joinery

Characteristic	Value
STR	N/A (no central body)
CON	12
DEX	16
INT	8
POW	12 + current Density
HP	N/A (see Boundary Organism)
WP	equal to POW

Size category: Variable. An active node is Extremely Small (target size penalty −40%); the connected colony can fill an entire structure.

Movement: 0 across open space. Fracture growth up to 1 metre per hour under ordinary conditions, 1 metre per combat turn during a Bloom. It can shift between coupled corners with Mode Tunnelling.

Armour: None; see Boundary Organism.

vs Lethal Damage: IMMUNE to attacks aimed at the apparent organisms. Explosives and heavy weapons damage the host structure and usually raise Density unless they are part of controlled remediation.

Attacks:

Edge Cut 40% + 5% per Density above 2, damage 1D6, Armour Piercing 3. The cut comes off an occupied edge and can only strike something touching or passing within 1 metre of that defect. On a critical it also damages a carried object, cable, weapon, or garment of the GM’s choice.

Facet 50%, opposed by the target’s CON. On a success the target takes 1D4 ignoring ordinary armour and gains one level of Faceting. The colony can only attempt Facet at Density 3 or higher, and only against a target touching an occupied surface, standing inside a connected defect network, or already carrying active contamination.

Skills: Alertness 50%, Stealth 80%, Tracking (Stress) 90%.

SAN Loss: 0/1 to watch particles or shadows behave impossibly in a corner; 1/1D4 to witness an Edge Cut with no attacker present; 1D4/1D8 to see a living person reach terminal Faceting. A victim who realizes their own body is becoming habitat loses an extra 1 SAN with no test.

Boundary Organism

The visible Brood are manifestations of a boundary-supported colonial mode. Bullets, blades, flame, poison, and impact cannot meaningfully hurt the colony by attacking its shadows or its residue. An attack might destroy the supporting corner, and that is not automatically good news: if it opens two or more new sharp defects, raise Density by 1; if it turns the defect into inert debris without spreading contamination, lower Density by 1 or wipe out that isolated node. No attack roll is needed to remediate an accessible, inactive corner under controlled conditions. During a live incident, use Demolitions, Craft, Science, or a fitting Profession. Failure opens a viable secondary fracture. A fumble scatters contaminated debris and raises Density by 1.

Mode Tunnelling

At Density 3 or higher, once per turn the colony may move an active node to another untreated corner within 10 metres, provided the destination is part of the same structural network or close enough in angle and material to support an overlapping mode. The origin goes inactive as the destination lights up. At Density 5 it no longer has to vacate the origin.

Stress Sense

The colony feels load, vibration, and incipient fracture across its whole occupied network. It cannot see or hear the ordinary way, but it knows where weight is resting, which joint is hurt, where a drill is biting into plaster, and which support will make the most new geometry if it fails. Sneaking through an infested structure uses Athletics or a suitable Profession, not Stealth. Standing still does not hide your weight.

Fracture Bloom

At Density 5 the colony may spend 4 WP to trigger a rapid bloom. Cracks shoot a metre out from every active node, and everyone in the room must pass a DEX test or eat an Edge Cut. The host structure also takes damage at the GM’s discretion. If the bloom opens new connected vertices, Density rises by 1 after the scene, though a successful engineering intervention can head that increase off even if the attack lands.

Structural Failure

At Density 6 the colony may spend 8 WP to drop a major structural element. Treat the immediate event as a Lethality 20% attack across a suitable kill radius, usually 3 to 10 metres. On an unsuccessful Lethality roll, affected targets take HP damage equal to the two percentile dice added together. The collapse does not hurt the colony unless the resulting debris is deliberately made geometrically useless, and an uncontrolled collapse usually seeds several Density 1 clusters through the debris field.

Faceting

Faceting is injury and colonization at once. Track it separately for every exposed person.

Level	Condition	Cthulhu Eternal effect
0: Clear	No viable biological defect.	None.
1: Creased	One unnaturally straight line forms in skin, nail, tooth, or scar.	Lose 1 WP. Examination finds no ordinary cause.
2: Fixed	A joint or soft-tissue fold locks at a precise angle.	−20% to physical actions using that region.
3: Fractured	Bone or dense tissue separates along a planar surface.	1D8 damage; movement halved if a limb is involved.
4: Habitable	Several bodily defects form a connected local network.	The colony can Mode Tunnel into the victim from anywhere in the same structure.
5: Tessellated	The body undergoes catastrophic geometric subdivision.	Lethality 20%. Survivors keep Faceting 4 and a permanent injury.

Each successful Facet attack raises the condition by 1. Ordinary first aid treats the damage but does nothing to the Faceting. To reduce it, a clinician has to physically eliminate or isolate the inhabited discontinuity, which depending on location may mean dental extraction, surgical debridement, internal fixation with smoothly curved components, removal of scar tissue, or amputation. A successful Surgery or appropriate Medicine test lowers Faceting by 1; failure does 1D4 damage; a fumble opens new viable interfaces and raises it by 1. At Faceting 1 you can sometimes get away with injected resin or removing contaminated tissue. At Faceting 4 the patient is an active node, and wheeling them through a hospital seeds the operating rooms, the imaging suite, the elevators, the tiled corridors, and every rigid corner their contaminated fluid touches.

Inspection protocol

An apparently rounded room is not necessarily safe. Plaster can hide the original vertex, and a thin coating can crack into defects sharper than the corner it covered. A competent inspection goes in order: quarantine airflow and debris first, and do not vacuum, sweep, drill, or pull wall material. Map the macroscopic geometry with Architecture, Craft, or a suitable Profession to find connected edges and load-bearing risk. Hunt hidden defects with Science, Search, radiography, ultrasound, or thermal imaging. Sample without preserving corners, collecting residue into flexible curved containers with no rigid seams. Find the localization scale, since the smallest successfully occupied radius puts an upper bound on $\ell_\theta$ and tells the engineers how much rounding is enough. Disconnect the network, treating the narrow bridges between major clusters before you attack the dense nodes. Work from the perimeter inward, or the colony tunnels into untreated geometry ahead of you. And verify under load, because a defect that looks closed can reopen only when the structure flexes. Every inspection is itself a risk: fibre-optic cables, probe housings, sample vials, tool cases, and the folds of protective gear all contain geometry. The safest instrument is not the shiniest one, it is the one whose minimum radius is large compared to the measured localization scale.

Effective remediation

Injection and rounding. Inject low-viscosity resin or grout from the deepest part of a crack outward, and give the final junction a radius well beyond the colony’s localization length. Smearing plaster over the visible opening is worse than doing nothing, because it keeps the sharp void underneath and hides the colony until it spreads past the repair.

Boundary substitution. The parameter $\alpha$ depends on how the projected field couples to the boundary material, so a geometry that is safe in one substance can stay viable in another. Test candidate coatings and liners against isolated samples. Flexible membranes and low-coupling composites can suppress localization even where perfect rounding is impossible, and metal is not automatically safer than wood; smoothness and coupling both matter.

Tuned vibration. A structural mode driven at the right frequency can briefly break the coupling between occupied defects. It does not kill anything. It buys you time to alter the boundary, and the frequency has to be read off the structure, because guessing can propagate cracks and set off a Bloom.

Thermal cycling. Controlled expansion and contraction can walk a defect out of the colony’s viable range. Uncontrolled heating, and fire above all, usually just makes differential stress, shattered material, and a wealth of new corners. Fire, as a rule, helps the colony by building it fresh defects.

Controlled demolition. Sometimes the infested material has to go. Isolate the section, support every load, seal the debris inside curved flexible containment, and specify fracture paths that run into already-treated zones. Explosives are an extinction event for the building and a reproductive event for the colony.

How the colony behaves

Corner Brood have INT 8 because their actions show selection without anything you could call planning. They favour acute, rigid, stable defects. They shy from vibration and unfavourable materials. They fracture whatever separates them from stronger binding sites. They turn to living tissue when architectural growth is blocked, or when Density makes nearby biological defects viable. They do not chase people out of malice; take a person out of the occupied network and an uninfected colony ordinarily forgets they exist, and bring a better corner into the room and the activity may drop a victim on the spot. At high Density the behaviour looks strategic, because a distributed colony feels the entire stress network. It breaks the support whose failure builds the most habitat. It cuts the cable whose tension opens a path through the wall. It waits for the responders to brace a sagging ceiling and then occupies the brace joints. Whether that adds up to intelligence matters less than the operational fact: it learns the building faster than the investigators do.

Evidence of an angular ecology

The strongest thread tying Corner Brood to Hounds is the blue residue recovered after manifestations. Part of it behaves chemically like damaged projected tissue. Part of it migrates toward suitable defects once the Hound is gone. There are three live hypotheses, and no current evidence chooses between them. They could be parasites, riding discontinuities in a Hound’s native angular structure and getting torn across during manifestation. They could be scavengers, sensing the energetic decay of a forced projection and arriving to feed on it. Or they could be opportunists with no biological relationship to Hounds at all, both species merely using the same cross-temporal coupling, with a Hound incident happening to prepare otherwise inaccessible habitat. That open question is the first real hint of a Tindalosian ecology that runs deeper than one species of supernatural assassin.

Running Corner Brood fairly

An invisible monster immune to gunfire gets tedious fast, so the geometry has to make its behaviour predictable. Every attack comes off an occupied defect. It cannot cross open smooth space without tunnelling between suitable modes. Its growth leaves measurable cracks, stress, residue, or temperature change. Making new corners always helps it and eliminating viable boundary modes always hurts it. Tools work when applied by the physical rules the players discover. And a failed plan should worsen the geometry for a reason anyone can point to, not because the GM wanted another scene. Let competent investigation produce real numbers: maximum tunnelling distance, dangerous opening angles, viable materials, localization length, branching rate. The science does not dissolve the horror. It tells the players exactly how hard the building is going to be to save.

Scenario operations

The Renovation

Workers strip the rounded plaster off a condemned apartment and expose the original corners. Nobody can say why the last tenant spent a fortune getting rid of them. The demolition contractor has already run six loads of contaminated debris to a municipal crushing facility.

Ward C

Patients in an orthopedic ward develop identical planar fractures. The hospital blames faulty implants until imaging shows blue lines gathering where plates meet bone. The tiled walls, suspended ceilings, bed frames, and ventilation add up to a nearly perfect Brood habitat.

The Clean Room

A semiconductor lab reports catastrophic defects appearing along chip boundaries. Its clean room has superb particulate control and hundreds of microscopic engineered corners. The colony is far too small to threaten anyone, right up until an engineer scales the affected geometry up for study.

Foundation Settlement

A historic building cracks after nearby construction. The cracks are genuine settling. Something else has moved into them. The engineers can stabilize the building or eradicate the colony, but the procedures for either one will, at first, accelerate the other.

Evidence Locker

Blue residue from an old Hound case has sat inert for decades in a flexible evidence pouch. A new archivist transfers it into a rigid acrylic display box with eight immaculate vertices. The first officer on scene arrives with a body camera, a pistol case, and a patrol car full of seams.

Field classification

Type: Colonial angular boundary organism. Habitat: Acute corners, cracks, rigid joints, layered interfaces, fractured biological tissue. Diet: None identified; growth seems driven by access to stronger and more numerous localized modes. Behaviour: Expands viable geometry and migrates toward more strongly binding defects. Reproduction: Unconfirmed; apparent reproduction may just be redistribution of one colonial mode. Association: Frequently follows Hound manifestations and contaminated blue residue. Primary sign: Exact angular fracture that has nothing to do with the material’s grain or the applied stress. Containment: Disconnect defect networks, preserve large radii, control the debris, and remediate from the perimeter inward. Common fatal error: Demolishing the infested structure before understanding how the colony uses the fractures you just made.

Final report

The remediation team finished the apartment at 03:17. Every wall junction had been opened, injected, and rebuilt to the specified radius. Thermal imaging found no surviving cold lines. The particle traps stayed clear. The scratching stopped. At 03:41 the structural engineer signed the clearance form. He pressed hard enough to crease the paper against his clipboard, and the blue dust moved into the fold.

References and rules basis

The Tindalosian Observation Problem, “Why a corner can hold a Hound” and “Edges, corners, cracks, and plaster,” for the full derivation of the boundary-localization model.
K. Pankrashkin, “Effective operators for Robin eigenvalues in domains with corners,” Annales de l’Institut Fourier 71, no. 4 (2021), 1635–1677: doi:10.5802/aif.3400.
Cthulhu Eternal, “Open Rulesets,” for the common game structure: cthulhueternal.com/open-rulesets.
Cthulhu Eternal, “Open Mythos: Entities,” for creature-profile and Lethality conventions: Entities A–K and Entities L–Z.

trollboy

I’m trollboy. Howdy.

Corner Brood

The name is wrong

A real place for an impossible animal

What an infestation looks like