Deathstalker Adamant is a level 18 elite named Forsaken imprisoned in Shadowfang Keep.
After Rethilgore has been defeated, Horde players can ask Adamant to open the gate to the rest of the instance. On the other hand, Alliance players can choose to kill Adamant.
See Shadowfang Keep NPCs.
Background
In preparation for an assault on Shadowfang, two deathstalkers were sent to the keep to gather intelligence. Days passed, and they did not return. Without their information, the Forsaken could not do a succesful attack. While scouting the Shadowfang Keep, Deathstalker Adamant was captured in to a cell guarded by worgen called Rethilgore. But Deathstalker Vincent, was slain by unknown person, and left lying alone in the floor.