Nexus War

The  Nexus War was a conflict fought in the frozen wastes of Northrend between the blue dragonflight a nd the alliance between the Wyrmrest Accord  and the Kirin Tor  of Dalaran . The war was instigated by Malygos the Spell-Weaver , the Aspect  of the blue dragonflight and Lord of Magic.

Prelude to War
Malygos fled to his lair in Northrend after the near-total destruction of his dragonflight at the hands of the Black Aspect Deathwing. The grief of losing his children to the treachery of one who was a friend drove him to madness, and he remained hidden away for ten thousand years, paying little heed to the rest of the world. He left that seclusion only once, shortly after the Second War, when the Aspects united briefly to battle Deathwing and end the corruptive influence of the Demon Soul.

 Malygos' madness and seclusion ended when Tyragosa returned from Outland with the nether drakes she had liberated in Netherstorm. Their nether-warped physiology helped Malygos recover some of his lost sanity. As he recovered, the Spell-weaver saw that magic had run amok throughout the world of Azeroth, and he blamed this on the reckless use of magic by the mortal races that had arisen in his ten millennia of isolation.

Seeking a solution, Malygos turned to the Arcanomicon, the map of all of Azeroth's ley-lines granted to him by the Titan Norgannon. Using the map to locate all of the world's ley lines, Malygos began to divert their energies into his domain, the Nexus, on the island of Coldarra in Northrend's Borean Tundra. They were then channeled through the floating rings of the Nexus and blasted into the Twisting Nether.

Movement of Dalaran; the War Begins
Malygos' actions led to rifts opening in the fabric of magic itself. This siphoning of the world's magic also did not escape the notice of the mortal races - particularly the remnants of the Kirin Tor, the elite magi of Dalaran. Realizing that Malygos had become a very real threat, the Kirin Tor used what remaining powers they had to magically transfer the rebuilt city of Dalaran to the skies of Northrend, right on the front lines of both the war with the Lich King and the coming battle with Malygos.

However, there are some members of the Kirin Tor who believed that allying themselves with Malygos would be far more beneficial. Betraying their own brethren to preserve their own power, these mages have been transformed and bolstered by Malygos to become the dreaded mage hunters. Their duties included killing or capturing those who wield magic without Malygos' consent, destroying magic artifacts and aiding in the redirection of the ley lines using surge needles.

Malygos' genocidal campaign also incurred the ire of the red dragonflight, charged by the Titans to preserve life. The Dragonqueen Alexstrasza: Aspect of the red dragonflight, allied her dragonflight with the Kirin Tor in their efforts against Malygos.

End of the Nexus War
The final blow against Malygos was struck thanks to the efforts of mortal heroes from the Alliance and the Horde, fresh from their victory over Sapphiron, the frost wyrm guardian of Kel'Thuzad in the heart of Naxxramas. As a member of the blue dragonflight in life, Sapphiron still carried on his person a key to the focusing iris at the center of the Eye of Eternity, Malygos' inner sanctum.

The heroes delivered the key to Alexstrasza, who enchanted it with her power. The key could then be used to open the focusing iris and lure Malygos into battle. With the key prepared, Korialstraz made the fateful request: Malygos must be slain, and the Heart of Magic, his most prized possession, taken from him as proof of the deed. With their orders in hand, the heroes entered the Eye of Eternity and confronted Malygos in the final battle, where they were joined by drakes of the red dragonflight. With the combined strength of the heroes and the drakes, Malygos was defeated.