This topic didn't seem to take under Rules Questions, so I thought I would try it under Advice.
I have an idea for a trap and I'm trying to get an idea of what the CR might be. I'm thinking at this point its actually two traps that are dependent on each other.
You have a 30x30 room with a door to the north and a small 20 foot hallway to the south. You travel down that hallway and you trigger the trap. The door shuts and locks. The room heats up to extreme temperature (1d6 fire damage per minute). A small fire elemental is summoned into the room every round for 10 rounds. This encounter/trap is dealt with when you either escape through the door somehow or disarm the trap (either option stops the heat and fire elementals and unsummons any already summoned elementals). If you wait it out, the trap deactivates after 10 minutes.
It seems like this trap is two pieces, the "elemental summoning" and the "heated room" piece. What is the CR for each piece? Or is there another way to build this trap mechanically?
The "elemental summoning" bit of the trap would just be the regular monster XP, right?
The "heated room" piece would be relatively low on the CR scale. It's basically just a minor hazard, 1d6 fire damage per minute means the PCs might take 1 or 2 d6 of damage. Most PCs are pretty good at escaping.
Difficulty of the lock/otherways of getting out. Just found out how long it will most likely take to get out. Use that to find the average damage. Use that and all the other trap's misc features to calculate its dc via the mechanical trap formulas. Then treat each fire elemental as an encounter that awards appropriate xp. At 1d6 per miniute this just sounds like a way to just burn some CLW charges
If you want to add some urgency to the encounter, have the damage increase each round. 1d4, 1d6, 1d8, 1d10, 1d12, 2d8, 2d10, 2d12, 3d10, 3d12, 4d10, 4d12, etc. Or just 1d4, 1d6, 1d8, 1d10, 1d12, 1d12, 1d12, 1d12, etc. And make the players roll the damage, taking turns as you go around the table. It keeps everyone involved.
It would add a time element to the trap, and it would give everyone something to do. The rogue would work on the locks, the fighter and mage would fight the elementals, and the cleric would be healing to keep everyone alive.
If it looks like the PCs can kill 1 elemental per round increase the number of elementals each round, or have 1d3, 1d4, 1d6, etc. elementals appear each round. Maybe have the players roll for the number of elementals each round, too. It would add to the drama because the GM wouldn't be able to fudge the numbers and can blame each other for high rolls and cheer each other for low rolls.
Yeah, thinking about it more it might be better to calculate CR based on the totals of the elementals and then either adding a modifier based on the environment or treating the heating room as a mechanical trap. I also like the idea of slowly ramping up the damage.
I don't mean slowly ramping up the damage. I mean rapidly ramping up the damage. :-D