The official GemStone IV encyclopedia.
The dybbuk is a piecemeal composition of horror, its mismatched sections of body coalesced into a whole that would frighten a banshee. The thing lumbers, managing to look clumsy and menacing at the same time. The skin is pallid and stretched, and in places, gaping wounds reveal worse atrophy than that evident on the abomination's exterior. Huge hands grope before the dybbuk's trunk, sweeping around it in flailing arcs and leaving no doubt that close proximity to the creature spells dire results.
Dybbuks do not cast magic or have maneuvers. They will sometimes be buffed by eidolons. It is typically best to leave the dybbuks for last, since the waern have a hand biting maneuver that causes stunned RT with the potential to disarm, and the eidolons mostly cast warding spells (such as Pestilence and Torment) and curses that push down TD or cause itching which will also disarm. Dybbuks have more hit points than waern and eidolon.
Due to the risk of being disarmed and knocked down in Bonespear Tower, it is common for magic users to bring cheap runestaves with Elemental Blade (411). It will sometimes happen that a waern bites your hand off, for example, where the wound and stun makes you vulnerable so that another waern bites your other hand inflicting disease rounds, and then the tower shakes knocking you to the ground in an especially vulnerable way. The elemental flares will sometimes instant kill the waern and dybbuks, particularly by blowing up their heads. Eidolons are non-corporeal.
When you are dead they will "rake ribbons of flesh from your face", which is a rank 3 head wound.
Behind the Scenes
Dybbuks are soul transference possession spirits in Jewish folklore. Their physical appearance in Bonespear Tower hints at the transmogrification of life in whatever they were before becoming undead, which is a recurring theme in Bonespear Tower as a subtle homage to The Broken Lands. In the mythology they were warded off by properly made mezuzah, which was writing at the top of a doorway proclaiming the divine lord of the house, which is embodied by the writing on the tower door. The mezics would presumably be cultists of Maleskari, the Demon Lord of Death and Undeath.
The subtext for their description is the apparent basis of Bonespear Tower on Clark Ashton Smith's "The Colossus of Ylourgne", where the dwarf sorcerer Nathaire and his students plunder the graveyards to construct a hundred foot skeleton, and then piece together flesh from the many bodies to cover the skeleton. Nathaire transfers his spirit into the colossus, and carries his disciples with them. The twist in the Bonespear Tower story is that the tower is made animate with the demon Maleskari, and the dwarf Bonespear apparently betrayed his cultists by attempting to become the demon.