It's been a while since I read Buffy S12, and I don't remember exactly what happened (so if I get some of these details wrong, I apologize), but I think what happened at the end of it was this:
Harth Fray was the twin brother of Melaka Fray: the Slayer far, far in the future. And since they were twins, a sort of mistake happened and instead of Melaka getting all of her Slayer powers like she was supposed to, the Slayer powers were split between them. Harth then got turned into a vampire and then became a Big Bad.
He then goes to the present and is fighting our heroes (as for why he went back in time to the present, I don't recall), and Melaka, and Melaka and Harth's older sister, Erin, are fighting him in the present alongside the Scoobies.
Harth has a giant demon army at his beck and call, and the Scoobies end up thinking that the best idea is to have all of the Slayers give their power back to Buffy (something they've done once before)--that way Buffy can be super strong again: and that way a number of the Slayers can run away, and don't have to die in this fight that seems unwinnable.
So they do that--and the only Slayers that still have their powers are Buffy and Melaka (because she's from the Slayer line in the future, and was called naturally).
...And then Harth steals the Scythe and ends up with all of the Slayers' powers (including Buffy and Melaka's--because the Scythe was originally used to create the Slayer, of course, so it can be used to take their powers away, too. And he chooses to do that).
Except he then starts seeing all of the past Slayers' horrible memories, and is unable to handle it--and starts going mad for it (more mad for it)--and as he's distracted, the Scoobies are able to push back and steal the Scythe from him.
And the scene of the Slayers' memories we see is beautiful and hella interesting in general. But the fact that the memory of Buffy's that we see is of when she had to kill Angel is so telling--she really never got over that (how could she?) And even here in Buffy season 12, it seems that she sees it as the most painful thing she ever had to through.
