In dNetHack, #chatting with the corpse when ready for the Quest reveals that it belonged to Dahlver-Nar, the first Binder, who became a binding spirit himself. Talking to the corpse not only allows you to use the down staircase, but automatically binds his spirit and teaches you his seal. There is no message indicating that his spirit has been bound, so Binders may be surprised later when they check their base attributes and learn that they have teeth beneath their skin.
As a quest leader, the corpse does not seem to be so much a mentor who provides the player with instructions and encouragement, as a mere mouthpiece through which an unnamed entity, or collection of entities, informs you about Acererak. After you are accepted for the quest, it does not say anything else—not even if you return victorious from defeating the quest nemesis, or stop by to visit it with the Amulet of Yendor.
The strange corpse has a single melee attack, but its speed of 0 makes it effectively sessile and unable to defend itself. However, its very low natural AC and high magic resistance make it difficult to fight.
There is no encyclopedia entry for the strange corpse, but there is one for Dahlver-Nar.