Ok. This is an old one so i'll see if it's even still out there. I remember there being another slayer (another buff? I'm not sure, might have been an original character) who accidentally jumped universes into Buffy and Spikes. This slayer was in love with Spike in her universe but right before she was pulled into this universe she watched him killed. She was his mate so when he died she slowly started dying as well. At the end spike has to drain her and send her back so that another slayer would be called in the other universe while she was still able to die with her spike.
Ring any bells? | 
