The Netherworld Plane
Sometime during the Gap, it's known that servants of Zon- Kuthon operated interstellar ships using shadow drives, a type of engine that allowed travel into the Netherworld in a means similar to how Drift engines operate with the Drift. Such engines remain in use today, though the plane sees much less traffic with Drift travel being less horrific on travelers.
The shadow natives known as kayals are humans who have adapted to the Netherworld's cruel environment over countless generations. The fiendish velstracs long ago fled Hell and established themselves in the Netherworld. They've had a crisis of faith following the amalgamation of Shelyn and Zon-Kuthon into Zon-Shelyn, and they hope to solve their quandary of whom to serve through greater mutilations upon hapless flesh.