You are right, my apologies. A lack of sleep and I misread the OP. I ended up focusing on the final half of the post and my brain made some stuff up.

The second bit of mine you quote is indeed not supposed to be related to OP. Its related to the idea that an encounter is supposed to take up resources (which include HP, spell slots, actions that require rests, items, etc). I still stand by what I say about it being far too weak. I feel at level 15, with no magic items, a mundane or 2 could easily take out the mummy lord quickly, and thus the party could expend zero resources to do so.

I'll make more of an effort in future to fully comprehend posts, particularly when I am tired.