Recently, while playing the excellent Dungeon Crawl roguelike, a thought occured to me:
Let's say the characters have no way back. Maybe they teleported here, maybe they were exiled here, maybe it's all a cosmic game, for whatever reason, once they step into the dungeon, they are there to make it through or die trying.
Below lies countless levels of underground hell, and they must survive through it all.
What sort of changes would be needed:
Obviously training rules would have to go out. When you have enough experience, you level up, learning "as you go" from the harsh environment.
Food and water becomes important. Suitable clerics can of course create this through magic, but we must examine other options as well.
Various lichen and similar underground plants may be edible. Unless there is a reason for such not to grow, any character may search for vegetation, finding a rations worth of cave growths on a 1 in 6 roll. A benign DM may state that searching for anything (traps, doors) will also permit a roll for plants.
These growths are unappealing but fundamentally edible (they form some nutrition for quite a few dungeon inhabitants). With every such ration eaten however, roll 1D6. A roll of 1 indicates the character becomes ill, suffering a -1 penalty to all attack rolls and saving throws. After every 8 hours, a saving throw against poison is made to see if the character shakes it off.
While sick, eating more cave plants will only induce severe nausea.
Keep track whenever a character rolls a 6 on the sickness roll. After 3 instances of rolling a 6, the character is now immune to the sickness.
We've covered turning slain monsters into monster burgers before.
When intelligent creatures are encountered, the DM should factor in the chance of them having edible rations.
If random rolls are desired, an intelligent opponent has a 2 in 6 chance of having 1D3 cave plant rations and a 2 in 6 chance of having 1D3 edible rations.
Of course, a dark, damp environment is not a good place for food to stay unspoiled. Every day, give a 1 in 6 chance for each ration to spoil.