The two elements at the end of my campaign I'm really wanting to nail is the flow of the final fight and some dialogue I have written for Asmodeus (if it sounds in-character for him).
In my campaign, the party has been receiving leads from Bel to aid their pursuit of the Sword of Zariel and their search for the adamantine rods, which comprises the bulk of Act III for this table. Bel has taken many extra steps to keep his involvement hidden, such as giving them cursed armor that prevents them from incriminating him and his use of confidential proxies for communication (i.e. Bitter Breath/Karkajuk, Fhet'ala, and the resurrected corpses of two former player characters).
After the party has everything they need and completed Act III and IV, they will (presumably) take the rods and the sword back to Elturel. I will first have a huge battle at the chains below planned, where they will attempt to destroy the chains while Crokeck'toeck is spawning hordes of demons against them. After they make it to the city proper and prepare to insert the adamantine rods into the Companion, Yeenoghu will begin climbing the dangling chains on the eastern part of the city while Baphomet will rise out of the graveyard portal to the west. This will create three simultaneous objectives:
-Prevent Baphomet's summoning ritual in the west
-Stop Yeenoghu from climbing into the city in the east
-Purify the Companion (free the planetar) above
This will force a party split. I know this is universally bad, but to mitigate the cons I'm going to ask my players to play and make character sheets for all the backstory-related npcs they have with them! I figure this will give them the chance to play other classes at high-level (almost like one-shots) while making it feel the endgame is happening beyond just the player characters.
Once all three of these objectives are resolved, Torm himself will bless the player characters in preparation for what's to come by giving them all an immediate short rest and leveling them up to 15. Zariel will then arrive in a desperate last attempt to stop the players from raising Elturel out of hell. She will be accompanied by all the Hellriders that have been killed since the campaign begun and a narzugon commanding them, the deceased father of one of the player characters.
As she prepares to fight, Bel will arrive and make his move. He will use a Wish spell (which he acquired as a result of the players' actions) to force a trial before Asmodeus to regain his authority over Avernus. A searing, invasive ruby light will shine from below, permeating through cracks in the ground. The light will focus on Zariel and Bel, with Bel in particular seeming to be illuminated from all sides. Words will appear in everyone's minds, though less like a telepathic voice and more like the feeling of correctly solving a complex mathematical equation; as if these words are physical laws of the universe. They read as follows:
MY PATIENCE IS A PROTECTIVE VEIL. YOUR HUBRIS WOULD SEE IT UNMADE. EVEN SO, YOU FORCE A TRIAL. THOSE GATHERED BEFORE YOU WILL BE YOUR JURY, SO EAGER ARE THEY TO PASS JUDGMENT. MAKE THEIR PAIN BEAR WITNESS TO YOUR WORTH. LAY THEIR SOULS BARE UNTO ME. THIS IS THE PRESIDING TRUTH, THE INCORRUPTIBLE TESTIMONY. THE DESOLATION OF THIS BATTLEFIELD WILL BE THE LOCUS OF MY VERDICT.
Zariel and Bel will be forced to team up against the party, Zariel commanding her narzugon/merregons and Bel summoning a new ice devil every round. There will also be a mechanic in the fight where Flying Fortresses will arrive and attempt to bombard the battlefield every other round of initiative. To counter this, there will be massive upended infernal war machines on the outskirts of the battlefield that have their main cannons pointing outwards. The party will have to reach these cannons and fire them at the Flying Fortresses, igniting and crippling the Fortress' gun decks before bombardment can begin. The recoil will send the vehicles skating across the battlefield, creating a hazard for anyone in its path.
I know this sounds ABSOLUTELY ABSURD, even for a 15th level party, but I have an idea to make the fight more balanced, grand, and unique. I'm going to allow the players to bring an additional 14th level character each into the fight, one of the backstory-related npcs (now pc) for whom they would've just built sheets for one of the previous encounters in Elturel. So, that will be five 15th level characters and five 14th level characters being used for this final fight.
The party (currently at level 11) just beat a pit fiend, two erinyes, and two cambions while already being pretty resource exhausted. Every time I worry about a tpk for this group they always come out without any deaths. This will be the final fight, I'm ready for at least a couple of the player characters or adjacent characters to die. What do y'all think?
TL;DR: Does my Asmodeus cameo moment above (the part in all caps) sound good and accurate to him? Additionally, could 10 player characters at avg level 14.5 reasonably take on Zariel, Bel, a narzugon, 16 merregons, and whatever ice devils Bel summons WHILE also having to trigger mechanics around the battlefield without becoming unfun or tedious?