Doom, in most stories, is described as the domain of The Architect and the meeting place of The Pawns. It is a large, barren landscape, consisting of abandoned or ruined cities, forgotten for eons across time. Doom exists outside of any known time or space and it believed to be a pocket dimension within The Void.
It is unknown at this point how doom was formed, but it is believed to be made of out of pieces of destroyed Architect Verses, which were partially transported to The Void during The Reaper's Harvest, by unknown means.
During The Uprising on Kryesor caused by The Glitch, Many Pawns fled to uncharted pockets of Doom. After the freed Pawns banded together (later to become The Brotherhood of Exile) they launched a large assault onto The Architect and the remaining Pawns under his control. This battle still continues to this day, and will continue for all of eternity, due to the lack of a timeline within Doom. However, some members of The Brotherhood fled through small tears of space time into Architect Verses such as Kryesor and Ravisman, becoming refugees who try to fit in with every day society.