We just finished our run of the campaign, and I was very happy with the finale. I figured I'd toss in the events here to help others idea-farm.
One tweak I made going into the final fight was that I didn't want the encounter with Jhavhul to simply be a case of them walking in and he attacks as a big angry bad guy (at least off the bat).
I did have a few things that occurred as side-effects due to the players' actions, and I love doing that the adjust things.
At the end of Kakashon, only one of the Proteans was left alive: Magiyawalla. The party had decided to kill both sides, but with Magiyawalla ("Maggy") broken free during the raid against the Shaitan, the group found her later on and made peace with her in order to get themselves out of Kakashon. They ended up leaving with her, Dilix, and her servants. Kakashon was saved, and it joined into the rest of the multiverse as just a small demiplane anybody can go to if they had the knowledge. The scroll was burned away during the escape as usual.
Maggy elected to hold back during most of the fighting going on in Chapter 5, mainly because due to her nature, she wanted to see how things turned out. She was free of Kakashon, but she was now trapped in the mansion. She was a sad depressed protean.
Then the curse on the building was lifted, Dilix and others stayed behind to watch the property since the PCs were able to buy it with the deed and bribe to the Vizier, and the PCs returned to the prime material plane with Maggy.
This is where things shifted a bit.
Now free, she could return to her realm, but I had her sense how "soft" the region around the Pale Mountain was. Due to her nature, she felt how reality was thinning out and during the battle of Kelmarane, she abandoned the group. Eventually, she managed to get an audience with Jhavhul.
Although they were enemies in Kakashon, she realized his wishing was damaging reality. If it was done too much, it was possible the material plane could be unmade here, just like what she wanted in Kakashon, and just like what she wanted for all of reality. So she joined Jhavhul's forces. (Being chaotic, she had no problem with shifting her views.)
Jhavhul got some early incite on the PCs and learned about their personalities: specifically, more of the PCs were not good people. Maggy made a recommendation and he listened.
When the party finally got to the chamber with Jhavhul, it was at 996 wishes done. He purposefully held off torturing one of the captives (Zastoran) to let the PCs have a single wish in return for them leaving. He even offered them his harem (since he had a bigger goal) and said he would no contest the loss of his home in the City of Brass.
This tempted most of the group.
Personally, I enjoyed this more, because it delayed this from just being a fight and done ending. Plus, with wishes being central to the campaign, I wanted to tempt the party with one. Even with the wish not being the final wish, they had less pressure thinking it would be their responsibility if he won. They just knew that regardless of the wish, Jhavhul could still veto it; and Jhavhul even told them he wouldn't corrupt the wish intentionally... as a sign of good will.
As the group was debating, the one good aligned character saw they were going to take the deal. Jarusha (gnome druid) let the others continue to debate, and she stepped back, turned herself into a mouse, and moved a little closer to Jhavhul (remaining far enough away from the lava tentacles).
She reverted back and made a wish that she had all of the items that Zara (another PC in the group) had at the time they were in Rayhan's house, in the quality they were then.
Jhavhul failed he sense motive... badly. He thought Jarusha was just robbing another party member (and others if the items had gone elsewhere in the meantime). He let all of Zara's things that he had at that time (most of which was sold or traded), appear around her. Jhavhul felt the power of the other two wishes go off and bring the count to 998.
While that happened, the rest of the party ran back in to discover what had happened. Immediately Jarusha scrambled through the items and found it.
I had to make a call, and I decided to go for it. Since Kakashon was now connected to the multiverse as a regular demiplane, the Scroll of Kakashon was no longer an artifact level item. It was just a portal.
Zara had the scroll in Rayhan's house most of the time.
Jarusha saw the ashes that were teleported, and she saw the ashes burn backwards into the scroll.
Maggy saw this happen and teleported back to her plane INSTANTLY.
Jhavhul was still relishing in being only one night away from victory and didn't see the scroll until Jarusha activated it.
Jhavhul, the party, and Zastoran (who was invisible and silenced nearby, in case Jhavhul needed to force the wish from him or use as a hostage over the lava) were all sucked into Kakashon.
The party appeared on the beach, near the Wayhouse, right where the party appeared before... plus the corpse of a dragon turtle they killed during their first visit.
All of the buffs Jhavhul had from being in his lair were gone. Jhavhul recognized where he was and screamed. Now the fight began.
After a kick-ass fight Jhavhul got to the point where in the text it said he would plane shift out to escape. However, he thought he was trapped in Kakashon still. He didn't know he could just leave, and so he desperately fought to take down another member (our orc barbarian Knee-Break died and several others were BADLY hurt). However Jarusha was able to jump forward and with a critical hit with Tempest sliced him up the midsection as far as she could reach.
Luckily they were able to rez Knee-Break, go after the big red bull on the island (another massive fight) before dealing with the rest of the finale. The oracle dealt with the pig demon that had initially survived their trip to the endgame. The sorcerer dealt with Xotani's heart, and everyone else went back to the mansion. The party broke up after that, for good and ill; but it was a great ending.
I loved the fact a player was able to completely change the setting of the finale, and the beach setting just felt more open; and as I said, the idea of letting a wish that proves to screws the villain over felt more akin to the tales I've heard... be it Aladdin or Wishmaster. It felt right, and I loved giving the players that ability to have a greater impact, more than just swinging harder and casting attack spells.
Thanks for reading the ramble.