The fic was a multiple parter and complete. It had a slightly darker spike kidnapping or taking Xander or something to that effect to make him his pet.
He got Xander to eat from his hand, gave him a things to keep him entertained in that was his alone and took them away when he misbehaved. He tried to sort of ease Xander into being a pet in a way.
And at once point Xander became ill but hid it from Spike and Spike ended up taking him to a hospital where it became clear he was respected by the doctros-called master and that'. At the end, the scoobies took Xander back but i think he went after SPike and wanted to stay with him.
I know it's not Xanpet or beutifully broken, and i'd love to re-read it.
ETA- got it! Thanks raphe1 :) it was piksa's Dark Surprises