An epic challenge, Prison of the Firebringer begins with wilderness locations and caravan attacks. It might work better for Ptolus if the wagons and outdoor locations get replaced with pack lizards and caverns. Cultists are behind these attacks as they search for a caster of lawful bent to force into casting the ritual to free the demon.
The story is that an academy of powerful mages summoned and trapped a Slaad Lord before they themselves fell. Slaad are creatures of chaos so it works from that standpoint, but in the adventure it is possible to negotiate with or even try to fool the powerful prisoner. To me, that aspect might rule out a Galchutt. Instead I would consider a powerful demon. Now some cultists and their powerful allies want to break the demon out.
Who are these cultists? Chaos cultists have been done to death so something fresh is needed. As written, one of the leaders is a Durzagon, basically a half-fiend/half-dwarf. That could make the cultists some wacked in the head dwarves. Initially Duergar came to mind, but they're lawful devil worshippers, not chaotic demon worshippers. Derro? They are chaotic evil, but are not in the 5e Monster Manual. Well, just dwarves that are nuts.
The player characters get dragged into things when they come upon the survivors from an attack who tell them what happened and ask the party to help rescue those that were taken by the Acolytes of the Hidden Flame.
The caravan could have been going from the surface down to settlements below. Now all that's left are a bunch of fried lizards and the dead delvers that were guarding the caravan. The wizard from the named adventuring group is missing and could easily be someone whose name the PCs recognize. The cloth and wine the lizards were carrying are as toasted as the lizards themselves.
Placing the location of the academy into the dungeons below the city means determining who this cabal of powerful spellcasters were. Let's keep things uniform, they were dwarves, the lawful sort, who in desperation summoned and bound a demon in order to fight off a Drow invasion.
Among the locations of interest in the scenario are an observatory. At first this seemed a bit tough to work with and then I just decided that there's no reason powerful wizards and the like couldn't have an underground observatory. Magic is magic after-all. The observatory is the lair of a Beholder, so things get tough fast and the Beholder is untypical, being able to cast spells.
The Beholder wants someone to get rid of the bonkers dwarves and will point the party to where they can be found if asked politely.
Other locations include another ambush site complete with a dead pack beast, also cooked. A passage with a blood trail from a wounded pack animal. A campsite above the passage from where some cultists and a demon will attack the party. Another camp where some cultists can be talked to and maybe the party will get jumped by a half-fiend Minotaur cleric and another demon.
The cultists are crazy, but not zealots so it is possible to get information those captured.
After all of that the group will reach the dungeon and may face some Fomorian giants at their camp.
Inside the dungeon are some rooms with ghost guards.
In the original module there are numerous Slaad, but I might replace them with various types of demons.
As things progress one might get the impression that the original occupants were lawful, but just as crazy as the cultists. They used plenty of undead to guard the place.
Turns out the demon prisoner isn't alone, his mount, a fiendish Pyro-Hydra is trapped here as well. It happens to be a Legendary creature.
Bazin-Gorag is a two headed demon, maybe you could make it an aspect of Demogorgon.
In hindsight, it might be best to keep the Slaad instead of demons, but still use the bonkers dwarves as cultists.
Besides defeating this epic opponent, the adventure provides options for other outcomes that have repercussions later.
