Sacrosanct
Legend
This is what I was getting at in my previous comment. In a prior campaign, I modified a Liche into something I called a Sea Liche (an Undead Lovecraftian Fish Person Wizard). The Sea Liche's resting place was in a temple on the back of a Leviathan. The Sea Liche could command the Leviathan to submerge, effectively drown/flooding everything.
Terrain, especially the lair, can really crank up difficulty with no changes to spells and ability.
Yep. A lich may or may not be a CR21, but if it's in its lair, it sure the heck is a CR22 or higher. Fighting an enemy that is effectively immortal and doesn't have to eat or breathe changes everything when you're on its home turf.