Help Balancing Final Ilvaash Fight

I’ve been running The Shattered Obelisk for over a year now with a party of six, and I’m starting to get concerned over the final battle being a massive cakewalk.

If anyone has ran this fight with a large group, then I’d appreciate any advice. I’ve been considering adding some cultists to the fight and creating a “random effects” table that will shift the landscape on each PC’s turn as the realm gets destroyed.

The battle with the main cultists has me a bit concerned too, but let me know what’s worked for your parties.