Various ideas to prevent powerful bones generated on a level’s upstairs from instakilling a player:

• Don’t generate bones if the player died on the upstairs.
• Going downstairs doesn’t use a turn, but going up does.
• When saving the bones, set all monsters’ movement points to 0.
• If the player died on the upstairs, swap the upstairs and downstairs locations.
• A way to “peek” up or down the stairs without going there.