Well, the idea I had was kind of fuzzy because I only had the main concept but if ever I had to develop my idea (which is open to reinterpretation of course) it would be:
1) A humanoid race, arising long, long ago and discovering how magic works, especially temporal magic
2) They used this magic to extend their life expectancy: they "accelerate" time around them (making all things to grow old quite quickly) and put it in their bodies to slow down the degradation process of their brains
3) Only their neural system pass time, but their bodies still decay, and they need their bodies, so they "embalm" themselves to keep the cohesion of their limbs (hence the "mummy" look)
4) Stealing time around them, nothing can live, explaining why they lives in the middle of deserts, because noone would care if sand lack suddenly a hundred of years. It will still be sand. It's ridiculously stable.