In the frame of my campaign, the Tomb of Horrors is actually the former tomb of their main antagonist, a mummy (who has risen and left the tomb). The demilich is a guardian guarding an item the party needs to wrest control of or destroy the mummy's army of sand soldiers.
So a time limit isn't hard to install - the mummy and his army are strong enough to take over a new city for each hour that passes or something similar.
What specific time limit do you suggest?
Wow- that's a bit more urgency than I expected. I had in mind something like two days, maybe. Not enough that the time limit itself became a problem, just enough to make it clear that they couldn't just take their sweet time about it.
But since your group is apparently both powerful and savvy, this might be a golden opportunity to do something I've always wanted to do, but never quite had a chance: the ticking countdown scenario. Impose an extreme time limit, like one to three hours, and track the time and their progress through the dungeon round-by-round.
The idea of reducing healing was quite brilliant, I thought.
Alternatively, you could make the Tomb a negative energy-dominant area (1d6 damage per round), so the wands and devotions are necessary to survive in there at all (while simultaneously requiring caster level checks to work), rather than being denied. In that case you should insert Dispel Magic traps in choice locations throughout the Tomb to counter Death Ward effects.
Edit:
I won't let them take 10 or 20.
I wouldn't prohibit PCs from taking 10 or 20 in general -on what grounds?- but (assuming you're mostly talking about Search and Disable Device checks) some things (although not all, probably not even most) should be too difficult to succeed at by taking 10, and with a time limit, taking 20 will be self-limiting since it takes twenty times as long. In many cases taking 20 won't be possible anyway because failure carries a risk or penalty -that of triggering the traps you're trying to find or disable. But if they're doing something like searching the lab for one of the hidden keys, there's no reason taking 20 wouldn't work.
