Nether Fortress 42 is a fortress located in the Nether. It is the location where Testificate is imprisoned and the military headquarters of the Nether army. After he accidentally escaped with a damaged Ender Pearl, Testificate fought and killed a Magma Cube and ventured inside to find Nether Warts to eat. It is hinted that Steve and Testificate previously fought beneath the fortress against a pair of Magma Cubes.
In other stories
In the Wielder Chronicles fanfiction trilogy by OutKookDrive on Wattpad, the Nether Fortress is Herobrine's Lair, castle, and possible residing place when not in the Overworld. It looks like a normal Nether Fortress, only much bigger, with a throne room and trapdoor over lava entrance. A potions laboratory is right next to the Fortress, where Herobrine brews potions like the Sangue Rosso. However, this was destroyed by a freak Ghast attack. In Book 1, Herobrine's Legend, Zach and Harry go to the Nether Fortress to rescue Nya. Zach kills Herobrine and is able to save Nya, who returns with them to the Overworld. In Book 2, Herobrine's Revenge, Nya and Zach are both brought to the Nether Fortress by Clare and Herobrine.