Mortiss

Mortiss are eyeless, unsegmented roundworms, two inches long as hatchlings and up to six feet as adults, ridged along back and belly. A ring of leathery tendrils circles the head, which bears a venomous stinger and can thrust out two pairs of opposed jaws to grip prey and drink blood. They sense by scent and vibration.

Ships catch mortiss by hitting egg casings, bringing infested goods aboard, or docking for some hours near an infested ship or asteroid. A colony eats 1d3 points of hull damage weekly and can spread inward to the decks and hold. After three or four weeks, burrow tubes break out on whichever side of the vessel gets the most sun, and the riddled timbers give way under heavy loads. Fire destroys an infestation; a cure disease spell kills a 10' x 10' nest.

Disturbing the burrows provokes attack. A head sting deals 1 point, and the victim saves vs. poison at -2 or takes 1d6 more and is deluded for 1d6 hours, misjudging dangers as larger or smaller, or hallucinating being elsewhere entirely. Each round an Intelligence check can see through one aspect, with a +1 penalty for each poisoned sting. Worms also latch on with their jaws to drain 1 point a round, and as many as six can strike in each five-foot square an intruder enters. A worm has 1 Hit Die, so colony Hit Dice equal worm count, and each point of hull damage grows the colony by 1d6 Hit Dice.

Mortiss turn light into magic that lets them tunnel wood and rock like soft dirt, excreting clay they build into coral-like castles above their burrows. An unnoticed colony encases a ship within a year and destroys the hull, and scholars suspect some asteroids and small moons hide mortiss works at their hearts. Most life is unwelcome, yet void scavengers, void krakens, void whales and void sprites lair among them, partly because colonies refresh the air envelope. The hermaphroditic worms fertilize one another's geode-like egg casings and fix them to hulls, where they hatch within a week.

A worm bores one yard per turn but must bask up to an hour in light to earn equal digging time, and it can store no more than an hour's energy. Every Hit Die covers a five-foot square of surface, and each 4 Hit Dice raise one ten-foot-square castle 1d6 feet high. A month under a world's gravity and heavy air kills them, so prudent crews inspect hulls often and land for a month or longer to purge them.