So, I'm working on this final EPIC battle, with PCs that are 20+Boon, and all have a gem in their body that turns them into a living artifact...(so spells that originate from a caster will work thru an anti-magic field...but weapons and armor will not).
We've have never played beyond 20, so I took it upon myself to piggy back on the end of Princes of the Apocalypse.
After they have killed the multitude of cultists, this will feed the undead army of my epic villain.
The story began when the previous DM ruined the story of a PC i had played years ago, and has been since turned into a villain (which was fine).. he separated a spirit/shared entity leaving him to fulfill his own destiny (something I didnt want).
So, I'm running the follow up campaign to 'fix' the error... and making him a continuous threat for ages to come (of course we will have to take years for him to return to his full strength, which is fine).
So I thought it would be a good time to play an epic level campaign-beyond 20+boon.
there will be 10 PCs, all with (each player playing 2 of their level 20 PCs whos stories are not complete).
I'm just needing ideas for an epic and memorable battle that will be talked about for years-in game and out

This is my first campaign and havent DMd anything that really mattered (one other time with this group, but it was a failure... I thought).
By the end of the campaign, the main PCs will have a tile that goes with guarding rips in reality and preventing OWC's (Other Worldly Creatures) from coming into this world. But, opening the world to other game world settings (Eberon, Greyhawk, Sci-Fi, Steampunk, Wild West...etc...) Mainly because we have played in this world for so long- our older PCs are NPCs and there arent many Legendary items left in the game, but also to allow for crossover characters from other realms and worlds.
At the end-The Boss (my Necromancer) will fade away for a multitude of years before showing back up.
But this time, The Boss used the PCs to leave a trail of death all thru PotA. (by using another Players character who has since ascended to a Keeper of Time ultimate Chronomancer to be locked in a struggle between Order (time) and Chaos (Cthulhu), he is biding the PCs time to keep Chaos from destroying the world, but the effects of Madness are beginning to show up in a multitude of places).
So, while all that is going on, my Boss is taking advantage of the time to create his world of undead that he can use to rule the world. (this is also after the Out of the Abyss, when a moon was summoned and raised the tide and killed more people-more potential for undead...I used this 'new moon' to be the be a place where Cthulhu was imprisoned and banished long ago by the old gods... guess what... he was summoned back

).
When the moon was summoned, it brought with it a propecy of the 4 horsemen (I found a great homebrew for the horsemen, who were all shown on 4 different tapestries in the cave at Lance Rock at the beginning of PotA... when they later went to visit that cave, the tapestries looked as though they could be walked into)
So, thats the backstory... trying to put it all together now...
I havent put together the Horsemen story
I havent put together the Boss fight
...struggling