Valhalla is a settlement founded by Isaac (the game's creator), to test bugs et cetera on. It is located in a place called Gods Glade, which is said to be in the Wilderness somewhere. We learn, from Isaac, that Valhalla is surrounded by at least 30 squares of Gods Glade.
The only person known to have found Valhalla, other than the admins, was a warrior named Elva. She returned with a hammer, and killed people with it. And never told anyone else where it was. Pity.