If I can add my tuppenceworth – I quite like the “Tomb Kings” aesthetic – ancient Egypt and all that. There may be an ancient desert race of Man wiped out by the Elves/Orcs/insert name here, and come back to wreak vengeance on the living in general and their arch enemy in particular. I think we’re talking Necromancer or Vampires here, but with the desert flavour.
Scenarios may include periodic wakening. Every hundred years or so, a Tomb Army rises from its half-buried temples to obey some long-forgotten order involving killing everything in a ten mile radius of a certain rock.
Or maybe the passage of an army, the digging of a well, the plundering of a tomb or someone sneezing rather loudly in an unfortunate place causes the Great Pharoah Senhotep IV to rouse from his slumber and campaign to clear his lands of the living.
Imagine hordes of animated skeletons and half-decayed, desiccated zombies with associated underworld nasties, led by ritually interred generals loyal to their King literally beyond the grave, clumping over the dunes in search of revenge.