Hell is the realm of demons. William J. Blazkowicz travelled there at least once to fight the Angel of Death in the The Death Dimension area of Hell and later when fighting the Devil Incarnate. Hell was also the home of the Harbinger.
B.J.'s descendants B.J. Blazkowicz III and Stan Blazkowicz would also travel to hell when defending the Solar System from invading demons that the Union Aerospace Corporation had summoned after opening portals to the realms of Hell.
Local legends in Wulfburg suggest that there are an extensive network of catacombs beneath the city some of which lead to the very gates of Hell. It is thought be ruled by the Devil, and the source of dark sciences and alchemy as opposed to the more benevolent and God inspired Da'at Yichud research.