From publisher blurb:
Your PCs driving you crazy? Return the favor!
Cthulhu Boss Stats has a complete 5e write-up for a CR30 campaign ender. A suitable fight for a whole party of 20th level heavies, Cthulhu can tenticle-slap the PCs down to size, shattering their minds as easily as his greasy claws crumple bone.
The write-up includes reality-bending legendary and lair actions, as well as a few adventure hooks to help insert Cthulhu into your game.