Spike dies in the desert

All I can remember is that it was a S5 rewrite and Spike ends up dying/sacrificing himself in the desert after they try to make a run for it to save Dawn.

There's a fic where Spike starts out in the desert but since I haven't read it yet I don't know if it is the one. Did yours start out with Spike dying/coming to in the desert?

I don't think so. He was trying to lead someone away from Buffy but knew that he would end up out in the desert when the sun rose, Buffy was out there too. I am pretty sure they were, effectively, heading out away from the building where the Knights of B had them trapped when they tried to escape with Dawn.

Was Dawn a teenage girl in your story?
A story I remember had Buffy pregnant (with Dawn / the key). Spike and Buffy make a break from the building surrounded by the Knights. Then when Buffy can go no further, Spike makes her as comfortable as he can and goes into the desert to face off against the Knights following them, despite the chip.
That story is Fin Amour by Angearia, found amongst others here on the Spuffy Realm and the Bloodshedverse.

Huh, I don't remember her being pregnant but I'll check that out because it sounds like the same scenario, thanks.

I think that is the one I was thinking about, thanks.