So, we just finished up the final fight and I wanted to share my thoughts and experiences.
The final fight was... rough.
The party bipassed the tower and just flew to the top, meaning that the cultists all joine din the for fight, making it extra hard.
Ariadnah has so many save-or-suck spells that they were bound to fail one. Our paladin got grappled by the Star Seed's projectile right off the bat, and was basically out for the fight as he could never break the grapple. Our sorcerer got turned to stone, Hilda (who i just reskinned to Weiralai) used destruction on our swashbuckler and killed him outright, and our rogue and paladin survived by the edge of their teeth thanks to the sorcerer's prismatic wall
Once the rogue realized that this was a losing fight, he and the Paladin decided to hope for the best with the wall. They allowed themselves to be hit by the wall, saved against a lot of the damage (though the paladin died in the process) and used the final bit of the wall to transfer planes, dragging the stone sorcerer with. I randomly rolled and they ended up in the Plane of Water, with the Rogue I ruled (as they had already lost so I let em have their small victory) eventually found land, was able to eventually get the dead paladin and the stone sorcerer back, raised them, and using wish were able to get the swashbuckler back.
Though the final fight was a loss, they just said they lived out the remainder of their days on the Plane of Water, just vibin.
MY thoughts on the final fight and campaign as a whole:
First of all, save or suck spells suck. I hated casting flesh to stone on the sorcerer, but that's what she'd do in that case, it makes sense. He was a big threat. But alas, that's part of the game.
I do wish Ariadnah was teased throughout the game more. I know Lowls is still really the "BBEG," but she's not mentioned at all aside from in Book 2, (and the tiniest bit of a retcon from the Pallid Mask in 6) when they think the woman they killed back then WAS the witch. Maybe have the party receive visions when approaching Star Stelea of Ariadnah and the Thrushmoore Vanishing, among other things important to her.
*REALLY* push harder than the book suggests to get the party to go through the tower. The book does its best, but a fly spell and some damn good rolls for the Yellow Sign meant that the party now had two incredibly hard fights going on at the same time.
There's a LOT of grapple in this adventure. Make sure you're familiar with the grapple rules (as familiar as anyone can be) or have a flowchart open.
I think the tone of the campaign was on point, lots of moments where our players would be uncomfortable (in an acceptable, appropriate way) with the descriptions of creatures or events. Book 4 fell a little short, and was my least favorite overall, but it was fine.
All in all, one of my best gaming experiences of all time. The year and a half campaign was a little sad on ending in a loss, but that's just how the dice decided to fall.