Initial movements towards a bestiary for THIS environment.
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Garden of Mouths
Colony organism that lives in holes bored into the iron beneath the surface of the water. Simple, 2ft long wormlike bodies with a keratin claw or tooth at the tip. They shoot out of their burrows en masse and 'chew' their prey by tearing it to pieces in the water. Once it's dead and ripped up they will use their bodies to hold what's left of the corpse against their burrows, and extrude soft mouth parts to consume it.
Each colony is made up of 2d6+6 worms, and a single pool may have many colonies.
They are HD0(1hp), can attack within 10ft, and deal d3 damage. Coming into range will trigger attacks from all living worms. They will prioritise eating a corpse over attacking.
Pattern Jelly
Ambulant slimes and jellies whose bodies strobe with a million different colours and patterns - a stable form that allows them to confuse and even disrupt the simple light sensors of the other things that live in the pools. They engulf their prey and dissolve them into absorbable nutrients with their acidic bodies.
Each jelly is HD1, slow, and attacks by engulfing the target. If you can see a pattern jelly you must make an easy WIS save (-1 for each additional jelly you can see) each turn to act normally.
Engulf: hits automatically. Each turn spent engulfed does d6 acid damage. If the jelly takes damage while it has prey engulfed, that damage is split between the two targets.
Thrasher Insect
Euphemistically called 'insects' because they have exoskeletons, many multi-jointed limbs, and a feeding 'head' with attached mandibles. There is no standard body-plan. Their armoured, whip-like appendages lacerate and tear at things that come near them, and they hunt by vibrations. Their jerky and unnatural movements are deceptively quick.
HD3, thrasher spin d8 x2, armour as leather, speed: very fast.
Light Beast
A predator-scavenger built around a single incandescent bioluminescent organ at the centre of its body. Within a certain distance the light does damage to biological matter, causing other animals to weaken, blacken, and eventually die. Its typical method of attack is to attach itself to a prey animal that is already too weak to fight back. Looks like a perfectly round glowing ball of light, with various translucent grasping limbs growing from it radially. These creatures, as well a bioluminescent algae, are the reason why many of the creatures that live in the bloom pools have developed light sensors.
HD1, no natural attacks, unarmoured, speed slow.
Gives off light like a torch - if you are within this light you take 1hp damage per turn. If you are within 5ft of them, this goes up to d4 damage.
Aquatic Ape
Rare, intelligent dweller in the pools. They look like white apes with translucent skin (you can see their internals) and no fur. Stat them as H1 White Apes with simple spears, knives, and clubs made from scavenged bone and keratin. They will hunt in packs of 4-8, and are one of the apex predators down here - depopulated pools are a telltale sign of their presence.
Paralytic Bloom
Coral-like animals whose long, barbed, nearly-invisible tendrils barbs deliver paralytic toxins. When prey has been subdued it is reeled back to the central mouth.
1HD, stinging tentacles (see below), unarmoured, immobile.
Stinging tentacles: save DEX when trying to move within 10ft (advantage if you are wearing chain or better). If you fail, save CON or become paralysed, with a save every round to end the effect. If you spend a turn paralysed within 10ft, you are moved to its mouth. If you spend a turn paralysed at the mouth, you take d10 damage.
Armoured Crawler
Aquatic pillbug-like insects with extremely tough armoured carapaces. They are slow but their legs and manipulator limbs are hideously strong, easily capable of tearing a human to pieces. They eat carrion and group together around sources of strong heat.
HD4, mouth d10, armour as plate+1, extremely slow.
Iron Thing
Long, sharp, splinters of iron that attempt to burrow under the skin of living things. In realty there are a tiny wormlike creature that makes use of the substructure of the iron maze to build their splinter-bodies. Once embedded, they reproduce by metabolising the body of their host into more Iron Things, resulting in many ambulatory iron splinters growing beneath the skin and trying to force their way to the surface.
HD0(1hp), burrow in d4 (-2 against chain or better), unarmoured, speed: as human.
If their 'burrow in' attack hits, the worm is now inside you. You have 3 turns to get it out before it lays its eggs. You can still attack the Iron Thing while it is inside you (you must deal slashing or piercing damage, bludgeoning and unarmed attacks won't do it), but any damage to the worm will be mirrored on the host.
If it manages to lay its eggs, you are infested. For d10 days thereafter d3 Iron Things will emerge messily from your body, doing d6 damage and 1 CON damage each as they do so. They will have new splinter-bodies made from the bone and cartilaginous matter of your body.