Arcanagos was a powerful blue dragon who traveled to Karazhan back during/prior to the First War to warn Medivh and the surrounding village that something dark and sinister was happening in the area of the tower. He sensed Medivh was being used by a dark force. Arcanagos pleaded with Medivh to abandon the tower before the situation escalated out of control. When Medivh refused to leave his residence, the blue dragon attempted to remove him by force. The two masters of the arcane fought briefly, trading blows, but the blue wyrm met his match in the corrupted Titan-imbued Guardian of Tirisfal. Burning from a horrific spell from the inside out, Arcanagos frantically flew off and crashed into the mountains of Deadwind Pass, dying a painful death, his bones shattering and falling down to scatter the surrounding area.
The spell Medivh used against the dragon imparted some of his own essence into Arcanagos. In order to divine information about Medivh, the Violet Eye has adventurers resurrect the dead dragon in order extract the arcane essence. Arcanagos appears, now as a fiery skeletal monstrosity known as Nightbane, and proceeds to attack those who summon him.