Barnom Slim stole power from him in some way.
[Chamber of Horns]
A pair of enormous horn-shaped stalagmites extend upwards into a ceiling of indefinite darkness, looming over a bronze and stained bone altar positioned between them. Sinuously carved grooves wind downward from the steepled altar, joining at the edge of a shallow depression that spans the length of a graded, uneven stone floor. At the depression's subjacent end, a bronze serpentine statue occupies a crude rune-carved alcove. A massive carved stone door interrupts numerous humanoid remains that adorn the chamber's irregular, convex walls. The stench of rot is consuming.
The thing before you may have been alive at one time, but only traces of its lineage remain. Its face is twisted and distorted, the flesh sickly and mottled. It wears some ragged garments, although the fabric once rich and lustrous is nothing more than shredded tatters now. It bears no expression or emotion, but a burning rage simmers in the crimson-flecked eyes of the lich.