Shastalith is a devil that formerly occupied a clone of Yoxxel created by the power of the Sallow Stone. She has a wide, disturbing smile and large, peering eyes.
Shastalith was placed into a clone of Yoxxel created by Deowyn Centhos in order to satisfy the Shadow Guard's plans to use the half-elf as bait for the Peas while keeping the real Yoxxel "safe" in another location. The clone was perfect enough that it managed to fool Phaelnah's Scrying spell, though it is unclear whether Deowyn hand a hand in such trickery himself.
Once the clone was defeated by Wander, it began to destabilize and melt into a pile of flesh, and a small avatar of Shastalith emerged from beneath it. Under interrogation by Wander, Shastalith revealed that she was simply working under Deowyn's orders, and showed fear at the prospect of retribution from Etarus. Unable to stay in the Material Plane without a vessel, Shastalith turned to ash, presumably returning her consciousness to a hellish plane.