Picking the right CR for a tier two party’s boss encounter?

I am currently writing an adventure for D&D 5e, character levels 5-10, and am trying to find the right boss.

A Mummy Lord is the perfect monster for the plot, however the CR (15) does seem a little high given the XP calculation rules on pg 82 of the DMG. At this point the party’s only encounter would be with 3 mummies, so hit points, spell slots, and ki should still be high.

Will a mummy lord devastate my party, or will it provide a dangerous, but not completely suicidal, boss battle?