Arcanagos was a powerful Blue Dragon sent by Malygos 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. 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.
Encounter with Medivh
Medivh: "You've got my attention, dragon. You'll find I'm not as easily scared as the villagers below."
Arcanagos: "Your dabbling in the arcane has gone too far Medivh. You've attracted the attention of powers beyond your understanding. You must leave Karazhan at once!"
Medivh: "You dare challenge me at my own dwelling? Your arrogance is astounding, even for a dragon."
Arcanagos: "A dark power seeks to use you, Medivh! If you stay, dire days will follow. You must hurry, we don't have much time!"
Medivh: "I do not know what you speak of dragon... but I will not be bullied by this display of insolence. I'll leave Karazhan when it suits me!"
Arcanagos: "You leave me no alternative. I will stop you by force if you wont listen to reason."
Arcanagos glows with a blue aura as Medivh casts a pyroblast that reflects off the dragon. Injured, Medivh stands back to his feet.
Medivh begins to cast a spell, weaving his own essence into the magic.
Arcanagos begins to glow red.
Arcanagos: "What have you done, wizard? This cannot be! I'm burning from... within!"
Arcanagos flies off into the distance aflame
Medivh: "He should not have angered me. I must go... recover my strength now..."