The Estherian Spectre is the ghost of Jessa's father, who was killed by the Wraith Lord.
After entering the Rotting Crypt, the player discovers him not far away from the entrance. Speaking to him will conclude the side-quest The Wraithring, and starts follow-up side-quest The Locket.
Although the Spectre is bound to the place in which he died and may never leave, he still shows concern for Jessa as a parent, instructing the player to prevent his daughter from entering the crypt.