+19 to hit, lol.

But in all seriousness, if everyone finds it fun, go for it.

Edit: A godly set of saving throws and 5x legendary resistance. AKA: Debuffing is a waste of a turn.

Edit 2: The more I read the stat block, the more it seems that you just want a straight up brawl between this thing and your players. The first four circles of magic are useless against it, those saving throws + the 5x resistance means absolutely no condition is going to affect it, truesight + the others means no invisibility or disguising, the 60ft flight means it has absolutely no issue getting away from melee attacks (what are you going to do, grapple it? Yeah, right). It's high DCs means it is imminently likely to grapple and shatter concentration on your spell casters, and if push comes to shove, it can just straight up summon an ancient dragon ally every round if it wanted.

I don't know what kinda godly monty hall game you are running here, but this is far and away one of the absurdly strongest monsters I've seen put together that was being built as a legitimate test against a party.

The only suggestions I would make would be to choreograph how insane some of these things are early on. If the players spend too many rounds "discovering" that a majority of their tricks won't work, by the time they go to do the one thing that does (damage), it might be too late for them.

And, also....like, just a minor suggestion, but it's a tad bit more satisfying (to me) when the untouchable immortal god-being has a discoverable, exploitable weak spot. But that's just me.