Nebulous
Legend
That is definitely one of the plot holes in the book. It's handled in a very simple game-ist way where the heroes are basically choosing their final boss battle by killing someone last.
The campaign books default assumption is that all 4 cult leaders are squashed and one elemental prince enters our world but is then destroyed. The four elemental nodes are shut down. The weapons are used to shut down all the nodes. More than likely the elemental prince that is killed is just set back for some time (do elementals die if killed on the material plane? I didn't think so, but I am uncertain). This is actually a pretty neutral ending. The characters have stopped the cults and the eye "for now". They've set back the individual elemental princes goals "for now". Sure, the valley is saved, but the entire adventure didn't change the game world. You are heroes, certainly, but what came of it?
My likely "bad ending" would actually require that all four princes are summoned and destroyed on the material plane. That was the Eye's plan all along. The cultists summon their respective princes who are in direct opposition and go to war. Since the cults have been pitted against each other and the princes are always destined to go to war with each other, the original plan was to weaken all of them. Then the double agents from the greater cult turn on their respective wounded masters with artifacts (the motes in my game) designed to steal their power. That power would then be used to bring something out of the eye for a real world ending problem. Obviously the players in my game have messed up this plan already by acquiring the motes, killing a prophet, and setting back (and unbalancing) the whole plan. However...
Windharrow is coming up next time, and want him to be a TALKER. A blab blab blab blabberr, right from the start, and part of this is so I can keep the plot moving for the characters AND for ME.
So going back to what you said, I like this. The elemental Princes HATE each other, especially the diametrically opposed ones. Once they are summoned into the world, they're going to rally their cultists and prophets (if alive) and wail on the other cults in a jamboree of fun that the PCs don't want to get in the middle of if they don't have to.
And they DON'T have to. Really, these bastards might just duke it out themselves. What if all the PCs have to do is wipe out whoever is left?
See, the cultists want to prove to the Elder Eye that "our Prince is the best" and the surviving Prince (whether Imix or YanC or Ogre or Hydra) can use the 4 elemental weapons in the Fane of the Eye and rip open the prison plane and await their reward...
...but as the 5th cult knows, there is no reward. The Prince is annihilated, and then the world is consumed.
Just brainstorming here, not sure if i'm headed the right direction or not. Any ideas?