Add multiple sub-branches of Gehennom, each containing a single demon lord’s lair. Killing them is optional, but each one that remains unkilled provides passive effects to thwart the player within Gehennom, especially on the ascension run. The mysterious force could be one such effect. The idea is that it’s not feasible to skip them all in a normal game because the combined effects are too difficult to overcome, but not necessary to kill them all either. You are able to choose which effects you want to put up with.