FormerlyHemlock
Hero
(Formerly titled "lich bat-cave")
I am making one of the ultimate bosses for my party to fight; a lich with an enormous hidden lair under a city. The problem is he needs it to BE ENORMOUS! He is storing hundreds if not thousands of corpses (including one of a dragon) in order to use for an undead army (I've already handled the issue of only being able to have so many undead at a time due to spell slot restrictions).
Liches in their lairs don't need to do anything special to overcome spell slot restrictions anyway. They recover spell slots with a lair action, which means they have functionally unlimited slots-per-day.