Okay, here's the encounter:
A 50 ft. diameter circular arena ringed by a 20 ft. wide ledge 40 ft. above the arena below. Opening from equally spaced apart points on the ledge are 4 metal "mouths" which contain within them piles of pure abyssal energy that release a negative effect onto the arena below while the "mouths" are open. There are only two doors into the room, the entrance at arena-level and the exit opposite the arena door and on the ledge. Both doors are open only while all 4 mouths are closed and have a portcullis over them otherwise.
On the ledges are 8 vrock demons, in the arena is a cannon golem.
4 of the vrocks have to constantly hold the metal mouths open, risking dropping and closing them if they're attacked or killed. Extra vrocks force the mouths back open if one of their own dies, but this lowers the number of them that have their hands free to make ranged attacks.
The vrocks open the mouths and thus close the doors as soon as all but one of the PCs are in the room, potentially trapping one on the other side of the portcullis (though this may actually be beneficial to a caster such as a wizard). The spell effect caused by the mouth then triggers, and the cannon golem leaps down from the ledge directly above the entrance in order to fight in the arena.
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My original idea for the spell effect was along the lines of crushing gravity, forcing Str checks to not be pulled prone, and fly or climb checks to attempt to reach the vrocks above. Alternatively I simply considered a negative energy field for damage each round, which of course the golem would be immune to, being a construct. Couldn't find an appropriate spell for either idea though, except perhaps something like mass inflict moderate/serious/critical wounds (incidentally, how does that spell sound for this purpose?). (Note: I just neat the effect to match what I'm going for, something that requires a save each round for an effect each round. Area of effect and duration can be altered to fit the arena and fight by way of the effect coming from a pseudo magic item.)
The PCs would be 17th level for this. Assume standard party of 4, at the moment a Fighter, Druid, Bard, and Oracle, but no guarantees that those four will all last this long.
I looked at create demiplane myself earlier and was somewhat turned off by the fact that they don't really provide mechanics for high gravity. The other two spells however, look very interesting.
Repulsion would be great, not only for preventing PCs from moving up, but also for keeping them away from the golem in the center of the arena (assuming the effect is in the center of the room, even high above them, they would have to save against it to move towards the center). I would just have to edit it to require a new save each round.
Overwhelming presence would have to be edited somewhat, as the ability drain effects don't quite fit what I'm going for, but the knocking prone part of the spell is potentially perfect.
I think I may go with Repulsion out of what I've got so far...
(Note on the demiplane: The dungeon itself is actually within the abyss. The ever-changing nature of which would pretty much already ensure that just skipping past something would be difficult.)
Can the cannon golem actually leap 40ft down without damaging itself ? Might want to give it some way to do that (gliding, maybe ?).
Interesting not about repulsion that I didn't think about until just now: if you put the center of the repulsion effect above the arena floor, then it will take a will save to stand up from prone since that would put you closer to the center. Kind of brutal.
Indeed. A flying wizard paired with a trip specialist could do similar.
Brutal. And I like it. >:3