Add a side branch that can only be entered by dying in a certain place such as the Valley of the Dead, or by life saving not working. When you die, you appear there with no inventory and have to fight your way out to return to your body. Monsters are also subject to this and appear in that branch after dying to the same conditions. However, it only works once - if you die again you die for good, even if you successfully got back to your body in the process.

Before you first enter the Valley, demons and undead (chosen from the graveyard monster algorithm, though perhaps excluding wraiths) continually spawn in the maze on the right of the Castle and try to path towards the upstairs. Also before it is set, in the Valley, awake demons and undead will try to path to the upstairs if they have nothing better to do.