In Dark Floors all ambient lightning is removed, the torches and chandeliers are also removed. The only light source to be found are from projectiles that emit, glowing parts mobs/bosses or explosive barrels. The respective mobtype (that is a smaller version) of the Miniboss Warhead can also be found in every room and emit light, this mob can be killed for regular loot. Dark Floors can only appear after the first level.
It is possible for Dark Floors to appear more than once in one run.