So this is how the Out of the Abyss book advises you to guide the adventure. However as always, you are the DM and can do relatively whatever you wanna do. My personality advice, since I'm planning on running this eventually and have been thinking about this as well, I would recommend running this battle the way its stated. But bear with me, for one, if the players take a step back from their expectations going into it, sure it may not be a one v one against a demon lord which is a daunting challenge alone, they are faced against several many demons lords. This can be an epic all out brawl between them, or you could run it where they from alliances, or hand control over to the players, there is options on options. But my second point, the fancy thing about demons is that if you don't kill them in the abyss, then their body turns to demon iccor and returns to their layer of the abyss where their body reforms. You could easily extend this adventure by coming up with another "dark ritual" or some other means of them finding a portal to the abyss and fighting and finishing off the demon lords once and for all. And also, a third point I just thought of, at the end of the demon summoning ritual where the demon lords fight each other, there will be a residing victor, possibly victors that they have to defeat. Perhaps Graz'zt and Orcus put a temporary truck to their eternal feud to destroy these adventurers who have proven their murth already.A good strategy, Daern, for sure but I'm still trying to discern whether or not this is how the adventure ends. And, if so, how easy it might be to restructure it so they fight each demon lord separately, all of those fights eventually leading up to Demogorgon. My players are not going to be happy about spending 15 levels looking forward to trying their hand at going toe-to-toe with each of those guys and then, once arriving at that point, discovering that Demogorgon is going to just do all the work for them anyway. To clarify, many players will love this, to be sure. I just would like to do something a little different and am wondering how easy it might be to change the story to involve separate fights.
Anyone have any thoughts regarding this? Thanks!

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.