Return to the Fallen Hero of the Horde with your findings.
Let us not mince words, [race]; my time here is limited.
You must destroy the triad of power which protects the demon lord, Razelikh. Doing so will not be as simple a task as freeing a damned soul.
You see, each of Razelikh's underlings wears a third of an amulet of Razelikh's creation around their neck. The very same amulet you will need in order to summon the arch demon.
Return to Trebor and tell him what is required to summon Razelikh. He will be able to instruct you further.