Help with creating a late game boss.

I'm making a CR 18 boss wizard/warlock guy basically he uses undead creatures a ton and is supposed to be super powerful obviously. What do you guys think I should roll for his HP? My original guess was about 21d12+100. If this helps any he's got 9STR 16DEX 12CON 26INT 22WIS 19CHA, I have a party of 4 players and he's able to summon any non-boss-like undead creature every turn. So he can make Flameskulls and the like as bonus actions.