That Game We Never Play
Lord Cyrus was (is?) the leader of the military arm of Duvan-Ku, here in the mortal realm. He lay dormant within the underground halls of the dungeon in Death Frost Doom. There, he awoke when he sensed living beings within the dungeon. Observing the party of adventurers as mist, he waited to appear until they had lost all hope of escaping alive. He offered a deal.
A mutually beneficial deal! He will keep the horde of dead from attacking, if they take him and his coffin to the surface. After that the adventurers are free to leave the area.