EL/CR for this Trap Scenario

[wolff96]

The party has upset someone with some decent summoning/binding capability.

The idea is to make a chaotic battlefield in the bottom of her gambling boat. I already have ideas for the rest of the boat, but the whole thing is supposed to be a trap for them -- it has out-lived it's usefulness at this point.

So she lures them into the hold with spells and a hostage. She then seals off the exit with a Wall of Stone, from outside.

She has previously bound an Invisible Stalker and it is in the room. Additionally, two Huge Water Elementals are outside the boat and -- at her signal -- start the boat sinking by opening valves to the seawater outside. The valves can be closed with Disable Device checks, though they re-open after 1d4 rounds (if all four are open the water depth increases by 1' throughout the hold each round).

So here's the way this 'should' play out...

Round 1) Wall of Stone. Two flood-valves open.
Round 2) One of the Water Elementals comes in through the opening to fight the PCs, aided by the Invisible Stalker. The second Water Elemental opens the third flood-valve.
Round 3) The Fourth Flood-valve opens. Elementals continue to attack.
Round 4) Now *both* water elementals and the Invisible Stalker are in the fight.

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All three elementals are CR 7, so that part is easy. The CR should probably be boosted by 1 or 2 for how much the terrain favors the Water Elementals, especially as the water gets deeper. If it ever gets to 10', the party is in real trouble, as the water elementals can use Vortex and really mess them up -- though it will reveal the invisible stalker. The summoner considers all of them expendable, so it doesn't matter to her.

What should the CR be for the "trap" of sinking the boat around them?

 

[SuperJebba]

I would say 13 or 14. IIRC, the monsters themselves are a CR 10 so the fact that the PCs are severely disadvantaged by the terrain and the elementals are benefitted by the terrain would make that number right in my mind.

Plus, depending on your use of the disable device skill, it may take several rounds just to close one valve.

Plus then the PCs have to escape the sinking boat and by the sounds of it, they are in the belly of the boat.

Yeah, I would say 13 or 14 depending on how they complete the encounter.

 

[Greenfield]

Of course, the first person to throw Protection from Evil on the party Rogue shoots the whole thing down. The summoned monsters can't touch said Rogue while they Disable Device on the scuttle cocks, and can't re-engage one if the Rogue is camped on it.

That spell is always the hole in any plan that depends on summoned, enchanted or conjured threats.