D&D 5E Help me balance a BBEG, please.

[Buck_]

So, we'll be having my homebrew campaign 'Grand Finale' in a few weeks with an all or nothing fight (no fleeing, no surrender) against the BBEG. It's going to happen at a big city square and the BBEG will keep the fight at ground level, for campaign reasons. My question is: how balanced is it for a end-of-campaign epic fight against a 9th level fully rested party? Any help is appreciated.

The party (all 9th level):
Elf Wildfire Druid
Halfling Celestial Warlock
Dwarf Battlemaster Fighter (shieldmaster feat)
Human Drunken Master Monk
Human Homebrew Paladin

Spoiler: BBEG

Edit: forgot about the paladin

 

[TheSword]

It looks good. Though condemnation seems a bit too powerful. Taking a PC out can always be frustrating. You could potentially do that 3 times a round... Charisma saves can be tricky.

 

[Buck_]

Oh, sorry, I forgot to write that he can only use each legendary action once on a given round.

 

[TheSword]

Good stuff.

 

[MarkB]

What is the area covered by Celestial Choir? Will there be bystanders getting minced by it?

 

[aco175]

He should last 3 rounds. I would add some minions to the fight and maybe give him some sort of weapon for when the PCs get resistance to radiant damage. It is nice that he has truesight and can teleport 1/turn.

 

[Buck_]

What is the area covered by Celestial Choir? Will there be bystanders getting minced by it?

Oops... it's 15 feet radius. Thanks!
No bystanders, the city is almost empty.

 

[Buck_]

Btw, I forgot to add the paladin to the party! There are five players, not four!

 

[aco175]

2 rounds now.

 

[Benjamin Olson]

Since you're keeping things at groud level where the Monk can go straight to melee I'd give it somewhat better Con saves, because stunning strike will use up the legendary resistances too fast, and a stun-locked creature is not a satisfying solo enemy for a party. If, on the other hand, it was flying and it was a multi-round struggle to get the thing in melee with the Monk then I wouldn't worry about it.