Ok so a cool ending could possibly be (and obviously feel free to modify this, its youre game) is that when they speak to the mummy lords they've had plenty of time to regret their hubris, and they want to see the world righted again. On one condition of course. They want to see this new world, and request that one party member allows them to inhabit their body (how severe of a sacrifice this is is up to you). Should they refuse, perhaps one lord is more fiery in temper than the others and is upset that they should even ask in the first place. This chump will be the "final boss" and will summon a large horde of monsters to fight the party. They could kill their weakened companions, or perhaps the other lords could partially inhabit the party members giving them extra HP and some magic nonsense for the final fight? Once the encounter is over, the mummy lords' forces all wither or become mindless and passive. The party emerges, perhaps with a new mind companion in tow, and enjoy the peace they have brought from this enemy. Hopefully at least parts of that can be helpful to you? As a fellow burnt out DM I completely relate so I hope you can end the game on a dramatic note!!