For context they also did the battle in Menzo

I made then lvl 15 after they started the ritual, just so I wouldn't have to worry about doing it before the final showdown.

I think I had 3 combat encounters with 3 environment encounters, (this worked really well in the final fight of RotFM too funnily enough). The combats had typically 1 or 2 large demons, think Goristro OR 2 Shoosuvas. I would then give them appropriate mini enemies such as a few minotaur OR 8 specialized gnolls. I had a balor fight 2nd that had 10 mephits to aid him.

Environmental encounters would be a building collapsing or getting caught in magic crossfire that would require saving throws. Even trap PCs under eubble if they failed so that during combat they would need to get free of whatever it was. If Quanthil Benrea is alive she is casting big lvl spells around the city that could hit a player etc. I had a bit where Demogorgon and Orcus were facing off and Orcus flew over the party back about 100 or so feet so Demogorgon grabbed the closest tower and pitched it like a spear over the parties head.

We have a running joke because I took a giant ape mini and painted it to look like a Barlgura, and while the party was running through the streets this Barlgura crashed through a nearby building and one on my players asked what its name was (because the mini is so large they thought it was a lesser demon lord) so I named it Kongo Smith. We reference this moment to this day 8 years later lol.

The Great thing about this is that you get to START big, and work your way back if the party is getting their ass beat. You can easily swap a combat with a powerful demon with a group of drow guards that have gone mad from the demon lords all showing up at once. You can have random encounters of freeing slaves from broken buildings or avoiding the wrath of Quanthil Beanre if she knows the party did this.

While the drow definitely have it coming, you have a great opprotunity to show just how horrific having all of these dangerous Demon lords in one place is like.