Bill Zebub
“It’s probably Matt Mercer’s fault.”
... but there is nothing like a meteor swarm, dissintegrate, or horrid wilting? That is a bit disappointing.
But PCs can already do those things! How boring. I much prefer a boss who can do a bunch of awful, terrifying things to which the players don't just think, 'Oh, yeah, that's XYZ spell; I have that, too."
My suggestion is that if you want something like meteor swarm or disintegrate or horrid wilting, make up your own cool ability based off one (or more) of them, but both more dangerous and more flavorful, and add it to actions or legendary actions.