Carapace

Known to some as the coffin shell creature, an unattached carapace is a soft gray sponge about a foot wide that lurks in the deep underworld. It can stretch tendrils from its body to catch a host and drag itself into contact. It bonds with nearly any vertebrate, reptile, mammal, or fish, but never with insects, spiders, octopi, or other invertebrates.

Unattached, it hides among other fungi or clings to cave walls and ceilings, with AC 10. Whenever it hits a creature, or bare living flesh strikes it, a sticky bond forms. If the fungus survives one more round, that bond hardens and the carapace starts spreading over the host's skin. Damage dealt to the spreading fungus is dealt equally to the host, but cure disease halts its progress harmlessly.

Full coverage depends on size: Tiny 1 day, Small 2, Man-sized 4, Large 7-10, Huge at least 14, Gargantuan as long as 30, since it converts part of the host's own mass as it goes. When finished, the host permanently loses 1 point of Constitution and gains a flexible, hornlike shell worth 2 points of Armor Class (a giant snake of AC 6 becomes AC 4) that leaves its joints free. The shell is now part of the host's skin: it cannot come off without injury, and cure disease no longer helps. The host grows visibly thinner and paler, often the first sign companions notice.

The shell guards its host well. For up to a day it regenerates 1 hit point per round to keep the host at no less than half its maximum, at the price of another Constitution point; none is lost if the day's healing is no greater than the host's Constitution and below a 3d6 roll. It absorbs poisons and mental assaults, so the host shrugs off poison, psionic attacks, and spells that affect the mind, and it drains a psionic host's PSPs too. The hardened shell is immune to fire.

A soft carapace creeps slowly over walls, ceilings, and almost any surface in search of hosts. Heat and motion attract it, while direct sunlight and negative energy repel it. On its own it can live for months on lesser fungi and slime molds; with ample food it grows to 2+2 HD and divides into a pair of 1+1 HD offspring. At this stage it is vulnerable to sunray and other fungus-killing magic, and to fungus eaters like burburs, purple worms, or giant slugs.

Once hardened, it gradually and fairly painlessly replaces the host's bones, becoming its exoskeleton. As it fuses with spine and brain, the host's alignment slides permanently to neutral on the good-evil axis, though lawful or chaotic leanings remain. When the host dies, fresh spongy growth devours the body over several weeks; the empty shell then splits to release one soft carapace per 3 feet of the host's height or length.

The carapace is believed to be a symbiont designed long ago by the faceless slime lord of the tanar'ri. Only desperate underworld creatures accept one willingly, though derro and aboleths sometimes force it on their slave-warriors.