The official GemStone IV encyclopedia.
The lost soul appears as the flickering shade of a normal humanoid, his face contorted into a soundless, agonized scream. His flesh slowly melts off his frame, bubbling, dripping to the ground, and his clothing turns into rags, dropping off in shreds, until nothing is left but a skeleton with hate-filled red eyes. Slowly the flesh reforms, the clothing regains its form, all in nearly reverse order, until the lost soul is once again whole. Then, horribly, he begins to deteriorate again.
Lost souls have relatively low health. They are non-corporeal undead, but their corpses do not decay quickly like most non-corporeal. They can be easily killed with front-loaded damage abilities (e.g., multi-strike).
Lost souls can dispel.
Lost souls are extraplanar beings.