Hi. I’ve been coming up with an idea for a boss fight. The story of my world is that a spell caused the universe to be very unstable, latching onto other universes to sustain itself. I had an idea for a boss fight where final bosses from other dnd adventures start showing up and fighting the party. I know about some, the black spider, the dragon in icespire(can’t remember if it had a name or not), Strahd etc. But what other final bosses would be good ones to include?

Icespire Dragon's name is Cryovain.

Tiamat, Chaulyimatar (Old Gnawbone)/Ebondeath (Probably didn't spell that right- Gnawbone is an ancient green dragon from the Icespire sequels. Ebondeath is the spirit of an ancient black dragon who possessed her), and Xanathar.

So for D&D: No Way Home, you also got Zariel, Auriel.

I'm porting Eclavdra of Eilserv into my campaign.  Lord Soth is a top-shelf D&D villain.  Artemis Entreri.  Vecna, always a good time.  Lord Robilar.  But all of these kind of depend on your players knowing/caring about D&D lore.  It kinda sounds like your focus is on 5e.  Have you considered pulling villains from movies or anime that your players would know?

If your using magic: the gathering content Bolas would be a fearsome adversary as would guild-leaders from ravnica.  Acerak (I am sure I misspelt that), any darklord from any domain of dread (especially Azlin though he is technically a dark power so have fun fighting a god / elder evil), Karsus, ended the netherese empire in the forgotten realms by using a 12th level spell to attempt to become a god, destroying all magic and ending the first netherese empire in the process, becoming a strange bloody creature, that one god who has an empire in Greyhawk and assumed a mortal form, Lolth the demon queen of spiders and drow, and the Raven queen (though technically she is morally ambiguous, she could be classified as evil).  For lesser bosses you have the guy from the sunless citadel, a guild master from ravnica, Klarg (the bugbear from the first dungeon in lost mines), a zhentarim leader and halaster (I think he made the dungeon of the mad mage though don't quote me on that).  If you want to add more without turning the tides completly in the ememies favor, consider having many of these fights "play out" in the background with some of them fighting good characters from dnd history such as Volo (comedic scene with him running from a group of goblins pursues), elminster, a hero from dragonlance, Bahumet, Mystara, and the circle of nine from Grayhawk.  

I hope my obsession thorough knowledge of dnd lore helped!

MidnightPlat Didn’t realize the connection lol. But yeah, essentially. It’s also an exscuse to have artificers and dunamancy and stuff. Can’t unsee it now.

Quote from WarmwaterGames >>

If your using magic: the gathering content Bolas would be a fearsome adversary as would guild-leaders from ravnica.  Acerak (I am sure I misspelt that), any darklord from any domain of dread (especially Azlin though he is technically a dark power so have fun fighting a god / elder evil), Karsus, ended the netherese empire in the forgotten realms by using a 12th level spell to attempt to become a god, destroying all magic and ending the first netherese empire in the process, becoming a strange bloody creature, that one god who has an empire in Greyhawk and assumed a mortal form, Lolth the demon queen of spiders and drow, and the Raven queen (though technically she is morally ambiguous, she could be classified as evil).  For lesser bosses you have the guy from the sunless citadel, a guild master from ravnica, Klarg (the bugbear from the first dungeon in lost mines), a zhentarim leader and halaster (I think he made the dungeon of the mad mage though don't quote me on that).  If you want to add more without turning the tides completly in the ememies favor, consider having many of these fights "play out" in the background with some of them fighting good characters from dnd history such as Volo (comedic scene with him running from a group of goblins pursues), elminster, a hero from dragonlance, Bahumet, Mystara, and the circle of nine from Grayhawk.  

I hope my obsession thorough knowledge of dnd lore helped!

 I was thinking they would fight one at a time, with a one-round break in between to heal and fight the guy summoning them. Thanks for all the ‘thorough knowledge’!

Your welcome!

Gar Shatterkeel and his two Sea Lions are the fun final boss of Locathah Rising

Frostmaiden. That's a fun one