Jourrael

 is a nightmarish specter, who has not yet appeared in person on the show.

Background
Referred to as the Caedogeist, Jourrael was a chosen assassin of Lolth, the Spider Queen, exalted through an arrangement with Asmodeus during a brief era of unified interests. Jourrael was a nightmarish specter who walked through walls and mountains like water. Known colloquially as the Inevitable End, Jourrael could not be completely destroyed unless the contract of the agreement between Lolth and Asmodeus was destroyed. In the bloodshed at the end of The Calamity, this attempt failed, so the body of Jourrael was sundered and sealed apart in secret. The heart was buried within the Lotusden Greenwood, and the skull entrusted to the elves in the Veluthil.

Trivia

 * The title Caedogeist likely comes from a combination of Latin and German, making closely translate to "Slaughter Ghost."