Ok, I haven't run it, but here's a basic rundown:
First things first, the trap DCs for detection & countermeasures. These seem to fall afoul of the DMG errata for DCs (which aren't in line with the DMG 2 trap DCs...). The DCs seem to be 5 (easy), 7-8 (moderate) or 10 (hard). I'd increase these in line with the DMG 2 guidelines to 5, 10 and 15 (respectively). Any DC already 15 or higher I wouldn't increase. This is especially important because the Perception detection DCs are compared to the PCs
passive Perception, making all of the information automatically detected for any party with at least one PC without a Perception penalty (i.e.: every party in existence). Chances are any given party will already have at least one PC with a passive Perception of 15 or more, but no point in making things too easy!
Right, so when the PCs first enter the room, I'd read/paraphrase the first boxed text, draw the map (omitting the P/S squares of course) and compare the passive Perception scores of the party with the moderate DCs (10 if using the DMG 2 DCs, 8 as written) to detect the steep, poorly mortared stairs and oddly hinged doors. Chances are good that at least one PC will notice. Note that I wouldn't use passive Perception scores to give them the second pieces of information under the stairs and ceiling detection entries - these should only be discovered with an active check after taking the prescribed action or asking specifically about them.
At this point, the PCs should be suspicious of the room, and should start making active Perception checks and/or attempts to bypass/disable the trap. I'd do this in a "round the table" fashion - allowing each PC to take one action and/or move their PC into the room. Once at least one PC moves into the room proper, read/paraphrase the second piece of boxed text, and compare the passive Perception of that PC with the easy/hard DCs to detect the abnormal ceiling and floor tiles. It's important to note the position of PCs in relation to the stairs and room itself.
Use the detection & countermeasures section of the trap stat block to run these "around the table" rounds - it should be fairly self explanatory (though note that the PCs should specify the action they're trying to take, and therefore shouldn't be able to - for example - open one of the trapdoors before they detect them).
Continue this until the trap is is bypassed/disabled or the trap is triggered by one of the following conditions:
- Two centre plates are depressed at the same time (I'd rule this as being if PCs and/or heavy objects are on 2 or more of the centre plates at the end of a "round")
- A PC attempts to open the opposite doors.
At this point, a number of things happen immediately (essentially the traps "surprise round" actions), as outlined by the trigger section of the stat block:
- Both sets of doors slam closed and lock.
- The centre plates begin rocking violently (no immediate effect).
- The stairs flatten effecting all characters standing on the stairs (see the description in the stat block).
- The floor pits open effecting all characters standing on a P square (see the description in the stat block).
- The ceiling spikes attack all characters standing on a S square (see the description in the stat block).
- The kruthik's emerge from the tunnel and attack (as described under development).
At this point make the PCs roll initiative and roll initiative for the kruthiks. Each PC/kruthik acts in initiative order with the usual in combat actions. On initiative count 15, the ceiling spikes attack again (see the description in the stat block).
Any PC starting their turn in the pit is attacked by the grinding ball (see the description in the stat block).
Any PC starting their turn on a centre square must make a Acrobatics check or be moved off (as per the countermeasures section of the stat block) - the trap doesn't specify, but I'd make this a free action at the start of the PCs turn.
Any PC starting their turn on a stair/ramp square acts normally but must make Athletics checks to move. Again the trap doesn't specify, but I'd rule normal move actions aren't allowed. As a free action a PC can choose to fall (as would failing an Athletics check by 5 or more), resulting in them sliding down to the bottom (as per the trigger section of the stat block). I'd also rule that moving onto (or around) either stair/ramp also requires an Athletics check.
At the end of 5 rounds (if the PCs haven't exited the room before then) the trap resets - woe betide anyone still in the pit at this point, because they'll continue to be ground until someone opens the pit and gets them out. Make sure you've got a counter of some sort to keep track of the rounds.
I think that pretty much covers it. It's very complex, but reads like it'd play out very well (other than the low DCs).
