Dawn is sacrificed and a spell prevents Buffy (and Spike?) from rembering.

Nan Dibbles's stories have Dawn disappear and everyone forget her. Spike has a tattoo on his arm that seems to keep reminding him of her existence.

http://www.allaboutspike.com/auth.html?au=247

that's not it. Buffy remembers Dawn but can't remember why she died. There was a spell on everyone and she remembers seeing (I think) Willow and Xander holding Dawn down while Giles killed her.

kspuffyk
you might be thinking of a story on TTH by Kimbclar called Immortal Slayer not spuffy but has willow holding rag over dawns mouth while xander and giles hold her down,then willow rewrites her memory of it