jgsugden
Legend
If you follow the recommendations in the DMG, a party of 5 PCs of 5th level will have only a handful of uncommon magic items. As others have theorized, the PCs may be a bit on the overpowered side of things. While any group can get real lucky rolls against most foes and manage to survive, it would surprise me if any 5th level party I've DMed against would survive terribly long against a Mummy Lord, even if it did not have Lair actions.
Putting that aside - I would come up with a backstory for that Mummy and then have them pursue the goals they had in life. That would likely take them across the globe to return to a place where they once lived/existed and trying to reclaim their seat of power. They might go there and fall to new heroes of that realm, might conquer it, etc... all possibilities open up interesting story questions (if a LG Paladin killed the Mummy Lord and claimed the near artifact weapon to use in their Righteous Cause ...)
Putting that aside - I would come up with a backstory for that Mummy and then have them pursue the goals they had in life. That would likely take them across the globe to return to a place where they once lived/existed and trying to reclaim their seat of power. They might go there and fall to new heroes of that realm, might conquer it, etc... all possibilities open up interesting story questions (if a LG Paladin killed the Mummy Lord and claimed the near artifact weapon to use in their Righteous Cause ...)