The Nether is one of the 3 demensions of regular Minecraft.
Gameknight and the NPCs traveled to the Nether to save Malacoda's imprisoned Crafters. They also returned after failing to save Crafter, and traveled through the portal to Notch's server (the one with The Source).
Gameknight and the NPCs battled Herobrine, Charybdis, and the blazes in the Nether. Herobrine was trapped in a computer and destroyed, but Charybdis lived.