The Nexus: The Nexus » Zones » WoW Armory Light

The Nexus is the name of a dungeon (as well as the first instance of the same dungeon). It is an ancient ice fortress found in the center of Coldarra, in Northrend's Borean Tundra. It is a "column of magical energy surrounded by levitating earth-covered rings" with ice caves underneath.

The Nexus (formerly Ice Caverns) is the level 71-73 wing of the Nexus Dungeon Hub. The dungeon layout is very open. You can go into different "wings", but the final boss Keristrasza, a captured red dragon, can only be freed from her ice prison when you kill the previous three. Click on the orbs around her to release her.