What is the Nexus in World of Warcraft

The nexus is located in the Coldarra in Borean Tundra, and is part of Malygos' grand scheme in revoking magic use among the mortal races. It consists of three parts. The Nexus houses the arcane machinery and servants of Malygos. The Oculus focuses the energies that are kept under control by the Blue Dragonflight. The Eye of Eternity is Malygos' lair, where the brooding great dragon lies in wait for careless interlopers.

The Nexus and the Oculus are 5-man dungeons, while the Eye of Eternity is a 10/25man raid instance. In order to start the encounter, one player needs to have the Key to the Eye, which is a guaranteed drop from Sapphiron in Naxxramas. On 10man, you get the 10man key to the Eye, while you need to kill Sapphiron on 25man mode to get the key to battle Malygos in a 25man raid.