Visceraith

A visceraith, or bone spirit, is a green-glowing brain with eyes and tongue, from which nerves and tissue dangle a full set of guts: heart, stomach, lungs and more. Only a wizard's magic joined with a fierce longing for life produces one, so none can be summoned or made deliberately.

Like a hermit crab, it usually lives in a humanoid skeleton it assembles with bone control, bones joined without tissue as in undead skeletons, brain in the skull, tongue in the jaw, organs in the ribcage. Any whole bone can be added if the frame can hold it. Entering takes a turn, leaving a round, and some build special frames, such as one with a beast's skull for biting.

There is a 95% chance it is wearing a skeleton, which gives it AC 2; bare, it has AC 7. Any intelligent being who sees it outside its frame must save vs. paralyzation or be gripped by fear for 1d4 rounds, taking -2 to attack rolls and a 2-point AC penalty. A frame with claws strikes for 1d6. With normal hands it may wield mage weapons, one in each hand at no penalty, but never armor or shields.

Its deadliest attack, bone control, can be used four times a day at a range of 60 feet. The target saves vs. death magic; on a failure a chosen bone snaps in a compound fracture for 1d6+4 damage. The shocked victim is immobilized, bleeds for 2 hit points each round until bandaged, and must pass a Constitution check or faint from pain for 1d12 rounds.

It keeps any spellcasting from life and may know as many spells as allowed, but never rises in level. Sleep, charm, hold and cold do not affect it, and fire hurts it normally. Normal weapons are useless. Enchanted blunt weapons of +1 or better deal full damage, while enchanted edged or pointed ones of that strength deal half. Holy water does not harm it, and it cannot be turned.

Shunning other undead, visceraiths seek the living, hoping to resume their lost lives. Robed and secluded, they may keep servants or companions and usually treat them well. They are not always evil, but they will do almost anything, murder included, to keep their secret and survive.

Being undead, a visceraith has no bodily needs and fills no niche in any world's natural order.