In the dungeons of Devarque, a vengeful wizard lay in shackles, plotting his escape. He shared his cell with a gibbering creature known as N'aix, a thief cursed by the Vile Council with longevity, so that its life-sentence for theft and cozening might be as punishing as possible. Over the years, its chains had corroded, along with its sanity; N'aix retained no memory of its former life and no longer dreamt of escape. Seeing a perfect vessel for his plans, the wizard wove a spell of Infestation and cast his life-force into N'aix's body, intending to compel N'aix to sacrifice itself in a frenzy of violence while the mage returned to his body and crept away unnoticed. Instead, the wizard found his mind caught in a vortex of madness so powerful that it swept away his plans and shattered his will. Jarred to consciousness by the sudden infusion of fresh life, N'aix woke from its nightmare of madness and obeyed the disembodied voice that filled its skull, which had only the one thought: To escape. In that moment Lifestealer was born. The creature cast its mind into dungeon guards and soldiers, compelling them to open locks and cut down their companions, opening an unobstructed path to freedom while feeding on their lives. Lifestealer still wears the broken shackles as a warning that none may hold him, but on the inside remains a prisoner. Two minds inhabit the single form--a nameless creature of malevolent cunning, and the Master whose voice he pretends to obey.
Source: The Game Website.