Revenant is also a hilariously sad challengeFor what it's worth, I agree with you on the o5e mummy lord and did my best to make the a5e version able to survive more than one round.
May I suggest you make it a Short or Long Rest like the Mythic trait? It seems to me that for most encounters the distinction is irrelevant, but I just feel like a Short Rest recharge is better.Legendary Resistance effects, like becoming sluggish, remain in effect until the creature takes a long rest (I've got a general rule in the Introduction to that effect).
Depends on if you want to allow the player group to escape from an encounter, take a break, and try again. If the creature recovers on a short rest, there's no gain from doing so. If it's limited to a long rest, the players have a chance to wear the creature down (even if HP is reset to full).May I suggest you make it a Short or Long Rest like the Mythic trait? It seems to me that for most encounters the distinction is irrelevant, but I just feel like a Short Rest recharge is better.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.