The Shadow King
A being obscured by ritual, dwelling in the inner wing of the Palace of the Shadow King in Graven-Tosk. The slaves of the servants of the courtiers of the King come forth and do his will.
The title is hereditary — sons are always born to the Shadow King. It's whispered Princes of that line disguise themselves as ordinary men wandering the ruins, engaging in games and tricking travelers, multiplying the miseries of their people.
His one true heir, his son Aldon, is currently missing — imprisoned in the Accursed Den by the cannibal warlock Fletcher. Without an heir the Shadow King will eventually be forced to hand his crown to his imbecile brother. A masked Seer of the King's court hires adventurers to retrieve Aldon (the scenario Rotblack Sludge).
The Line Before Him
A "black statue of the one-eyed King Lenard II" stands in the Statue Room of the Accursed Den — presumed prior Shadow King, anchor of the hereditary line. The statue's empty socket is the bloody key that opens Aldon's cell. Lenard II is named only on the plinth; the line between his reign and the present King is unwritten, but his statue still guards what's left of the bloodline like the line still owes him.
Palace of the Shadow King, Graven-Tosk, Aldon, King Lenard II, Accursed Den, Fletcher, Rotblack Sludge
The current Shadow King is never physically described — kept deliberately obscured by ritual. His brother is named only as "the imbecile brother." The Princes-as-tricksters legend is unused in any canonical adventure. What the ritual that obscures him actually does — and whether it is what makes him the Shadow King — is open. The lineage between Lenard II and the present King — generations, names, what happened to the other eye — is a gap.