I hope it goes well and sorry about Garl
Ha! No worries.
As it is, we played for about six hours straight and the fight is still on. Original participants were Demogorgon, Orcus, Graz'zt, Pazuzu, Obox-Ob, Dagon, Malcanthet, Bjornganal, and Yeenoghu.
I know you did not design these guys to brawl, so "game balance" was never an issue WRT them fighting each other. In the interests of keeping it interesting, though, I ruled that some mystical hoodoo about the summoning ritual and the faerzeress suffusing Menzoberranzan made ALL the demon lords' weapon attacks count as +5 weapons -- just so the little guys had more of a chance.
While I really wanted to use their summons, for my own sanity (and because in most cases the summons wouldn't really affect combat much), I similarly ruled that the ritual and the faerzress hoodoo messed with their ability to summon thralls and Gate in allies.
Malcanthet was the first to fall, killed by Yeenoghu, in part because her player did not really know what to do with her. Obox-Ob fell to Orcus next, his abilities not really able to do much to the other demon lords (although his ability to control all vermin within a mile radius gave the drow of Menzoberranzan trouble that cannot be overstated).
Yeenoghu, unable to fight his worst enemy Baphomet because no one chose to play him, killed Bjornganal instead after Orcus managed to knock the titanic goristro to the ground and stun him somehow. He then went after Orcus, who remained on the ground and kept fighting him until the lord of gnolls was dead.
Meanwhile, Demogorgon was getting tag teamed by everyone else but Dagon, who was focusing on Graz'zt. Pazuzu was, as I suspected, spending the whole time doing strafing runs with his breath weapon and dishing out a large amount of damage to almost everyone without resistance/immunity to acid and poison. Eventually Dagon had no one to attack but Demogorgon so he "betrayed" him and eventually killed him (but more on that later). At that point there was simply no one he could reach -- truth be told, Dagon was not a good choice to play for this, as the combat was in a city and out of the water, but the player had really been interested in him. Had the fight been underwater things would have been very different...
By the end of the night (we broke around 2 am), it was down to Dagon, Graz'zt, Orcus, and Pazuzu. Dagon managed to get ahold of a flying Graz'zt by teleporting over him and falling on him with a legendary action just before his turn, then grabbing him with his tentacles and swallowing him. Ironically this helped Graz'zt out as he is immune to acid damage and was planning to chill out in his gullet regenerating while gutting him from inside. Pazuzu and Orcus (who was very wounded) figured this out and worried about a fully healed Graz'zt emerging from Dagon's corpse after they had finished savaging each other -- maybe Dagon was crazy like a fox after all, because his swallowing Graz'zt prompted Orcus, at least, to get within range of his tentacles in order to attack him.
We discussed the likely outcome after packing up, and decided that Orcus was likely to die next, followed by Dagon, leaving a mostly-healed Pazuzu and Graz'zt to slug it out. I pointed out that Graz'zt had the advantage, as Pazuzu had to get within range of Angdrelve's acid jets in order to breath on Graz'zt, and all Graz'zt had to do was ready an action to shoot two of them (which is one action for him) every time Pazuzu got close -- Pazuzu's breath didn't do a lot to Graz'zt compared to what Graz'zt's acid blasts could do to Pazuzu.
Then Pazuzu's player (who is probably the most skilled powergamer of our group) pointed out that Pazuzu could use Wish to bring himself to full health. I realized that Demogorgon could have done that, but I had been playing him while running the whole combat (his original player had to go home) and I had forgotten.
We might pick up where we left off this weekend, just to see how things really end, and if we do I may have Demogorgon show up at full health, having somehow used both his Wish and Time Stop abilities in order to do so (maybe Time Stop got messed with by the faerzress so it instead sent him forward in time, while giving him time to cast Wish and heal himself to full).
A few comments in no particular order:
1. Orcus is healed by necrotic damage, and he can do a ton of necrotic damage with his attacks. Can he attack himself and heal that way? If he can, he is stupendously powerful and I think he very much outclasses the other demon lords. I ruled that he cannot, but felt strange about him not being able to cast Cause Wounds on himself to heal himself if necrotic damage really does heal him. Maybe it should be spelled out in his description?
2. I think Pazuzu is still a bit too powerful against the others even if I don't allow Soul Corruption to affect demon lords. Maybe it's just the combat's setting -- Menzoberranzan's cavern is 1000' high and gives him plenty of room to fly in -- but I think his speed is too high at 240. If he had a longbow he'd be nigh invincible. I also think that maybe his breath weapon should be on a recharge, because he had little reason to actually use his sword in combat against the others when strafing with his breath weapon was so much more effective when risk to himself was factored in. That's mostly what he did all combat.
3. Yeenoghu needs a longbow, like I said. I gave him one. He used it once, against Orcus or maybe Malcanthet.
4. I feel bad for poor Dagon but again, the fight would have been very different if it had taken place underwater...
5. Concentration spells -- such as the one required for Graz'zt to stay flying in combat -- have a mechanic that breaks down once you start taking over 50 or so hit points per hit. A blow that does 100 damage forces a DC 50 Concentration check that not even the demon lords have any hope of making. Not sure what the remedy is here but it is something to consider. I think legendary resistance can help with this somewhat, but... Maybe truly epic creatures should come with a feature that helps them in this area, for when they clash.
6. The mechanic for demon lord spells being treated as "6th level or higher" should be parsed less ambiguously. I think you mean that they should be cast as 6th level (or whatever the level is) spells if the spells are ordinarily of level 6 or below. That's how we played it anyway, but not before some debate.
That's it for now. Thanks again for some great conversions!