It's call Schemering Sintel on fimfic. Spike is kidnapped by a dragon and Twilight goes after him. She spends years trying to do just that. During that time she goes from an innocent filly barely surviving her first bounty hunt against wild dogs to a bad-ass assassin. this lovely piece of artwork reminds me about especially the knife. Before she secretly leaves Ponyville Pinkie gives her a cooking knife.
It's a sad story but a good read.