Stupid Boss concept I've committed myself to

Title basically. I've begun planning a campaign with the goal of destroying the egg of a ludicrously powerful, CR 90, 3.5e time dragon while it weaponizes the butterfly effect against you.

I need a final boss. Now, under no circumstances can they beat this thing in a fair fight. It was never made to be fought. Buuuuut, it could make a pretty good gimmick boss fight!

The dragon can barley be touched by most attacks, and can't be affected by lingering spells. So I imagine, in between regular attacks (this thing is perma-hasted too), it goes back in time, butterflies, and changes the world in the present to try and gain the advantage. But each arena change has a way to damage it integrated. 

Changes it to a dark fantasy Conan-like world? Oh hello castle siege with convenient ballistas.

Gothic horror dark crystal deal? Unfinished magic ritual. Nice to see you 

Scifi future? Turns out this plot of land is perfect for a mech docking station

Cyberpunk world? Hiii godlike ai who isn't cool with this giant dragon destroying its city!

The win condition isn't killing it, that's definitely off the table, it's breaking its egg and maybe wounding it

If anyone has other suggestions for ways to damage this thing, feel free to tell me

You've essentially created a "read my mind to solve the puzzle" fight. This concept seems like it would be too confusing for players to understand. I don't understand any of your posted "solutions," and you've spelled them out. How are players who are stressed and in the grips of adrenaline going to suss the right answers out?

It sounds like you're expecting players to understand that they're capable of warping reality, which is a tough enough thing to convey even when the entire genre isn't radically changing each 6 seconds. How you expect players to be familiar with all those different genres and expect them to pull out a solution is beyond me.

EDIT:

"If anyone has other suggestions for ways to damage this thing, feel free to tell me"

Looking at this line again, I'm going to just outright say that approach is wrong. You're asking for specific solutions, and what a DM should be doing is putting forth scenarios without a single "correct" solution in mind. Instead, let players come up with a creative solution, and go with it, seeing if they roll well enough to accomplish it.

Quote from Stabbey_TC >>

You've essentially created a "read my mind to solve the puzzle" fight. This concept seems like it would be too confusing for players to understand. I don't understand any of your posted "solutions," and you've spelled them out. How are players who are stressed and in the grips of adrenaline going to suss the right answers out?

It sounds like you're expecting players to understand that they're capable of warping reality, which is a tough enough thing to convey even when the entire genre isn't radically changing each 6 seconds. How you expect players to be familiar with all those different genres and expect them to pull out a solution is beyond me.

 Well, my group is pretty good at improvising, and the puzzle solutions are kinda just around. The dragon butterflies, and boom! Now the arena is a castle siege with siege equipment at hand. Also, I don't like to do it, but I might just tell them the gist of the puzzle beforehand.

If I’m this powerful being who the PCs can barely scratch, why am I going to go through the hassle of time traveling in the hope that I’ll create a scenario where they lose? Why not just walk up and kill them all? It’s a pretty big plot hole. It’s one of the reasons they don’t stay out gods (and what you are describing sounds like god-level powers) in this edition. 

I’d say don’t have the players interact with this being directly. You could have this being act like a cuckoo bird. They sometimes lay their eggs in the nests of other birds, in the hopes that these other birds will care for the chicks.

So our god like thing has tricked someone else into watching its egg. A different dragon is an obvious choice, but it could be anything. Heck, maybe it’s egg looks like a magic orb and a wizard has it. Or it looks like a big piece of worked stone, and it’s embedded in the walls of the kings castle. Or it looks like a 10gp agate and it’s on a ring someone wears, thinking it’s a kind of cheap piece of jewelry. Or it’s a curiosity in a museum somewhere, or part of a powerful person’s art collection.
Or, of course, it could have some cultists guarding the thing. And something with godlike power, but who isn’t a god, would likely have some warlocks running around serving it.

I like the warlock idea. Maybe have a rouxls kaard/team rocket antagonist be a warlock of this thing. I already had the idea of the dragon slandering the party through ancient prophecy, this is just one step further. 

I could have it be a hollow Knight/radiance situation where the warlock is technically the final boss, but their mind and body are being replaced with the dragon's as it floods them with power, but that might fall flat, cuz the dragon is the main antagonist, and this is just some annoying guy. and I just ran an adventure where the final boss was a humanoid that was hopped up on unearned power and lost its body over the fight, and I don't want it to feel samey.

I also do want to run a big spectacle boss fight, with a gimmick to make it manageable, but there were some valid points brought up that it might be too obtuse

Who says a warlock has to be a humanoid? There are plenty of intelligent monsters fully capable of making a deal with a powerful being. 

I also do want to run a big spectacle boss fight

The problem with what you have proposed is that the players basically don't matter at all. The dragon is just creating it's own destruction, the players could just sit in a corner and watch it play out and it wouldn't matter. 

If you want a big spectacle boss fight with a time dragon, then (1) I would nerf the dragon so it is kill-able, and (2) have part of the plot be the party learning how they can tether themselves to the dragon so they are taken along with it whenever it hops time. That way you have a relatively standard combat where the players use their characters & their abilities to defeat the dragon, but get the spectacle of changing the map & have random weird things happen for different stages of the fight. This would also lend itself to a crazy epilogue where the player characters will end up stuck in whatever time period they kill the dragon in.

Alternatively, if you want the time dragon to be unkillable and it is just the egg that matters, then the players never face the time dragon, but instead find & recruit / obtain some kind of item that allows them to travel in time as well, and thus the "final combat" is not a combat at all but rather a chase through time as the player character hop through time trying to get to the egg before the time dragon catches up to them in a particular time period. In this case the majority of the campaign is tracking the history of the egg so that they know where to go in each time period they jump to to find the egg & destroy it.

The latter idea of a time-chase could even be the entire campaign now that I think about it. Rather than having the party travel between different locations use the same location but at different time periods. Framing is there that a time dragon has hatched & is messing with the history of the world, the party is recruited by the time-police to go back in time to find the egg & destroy it so that the dragon never hatches, however the time dragon is hunting them down so they have a limited amount of time in each time period to try to accomplish their goal.

Hmmmnnn

The "maybe the egg is in something else" gave me the idea of the dragon starting a religion about itself in the past, and keeping the egg in a reliquary.

The main campaign as I plan it is traveling to the egg (which needs to steep in the correct temporal energies to hatch properly. DND time is a better environment for growing dragons), while the dragon, too prideful to face them directly, butterflies to make things worse for them. Making a religion against them, making sure a lich takes over the town they're in, moving rocks so they trip, giving characters fond memories of itself, eventually working up to changing the entire world's genre into cyberpunk, or dark fantasy, etc. (it'll just keep it like this for a bit, bc the egg needs the right energies)

Hell, maybe there are time police for a little bit, but then the dragon prevents their formation (bc this thing is basically a god, but also very much a created thing, and survived reality erasure, the gods don't really like it, and they like its egg even less, so the questgiver is an angel, who manages to give the quest, and protect them from being directly harmed by it's time shenanigans before the dragon prevents that religion from ever forming)

Hmmmnnn... Non humanoid warlock... I'll have to look through my books to find one that looks good in robes (for the religion angle)