After the Crystal Empire incident, Sombra was sent to Ponyville to be reformed. Pinkie Pie, of course, decides to take advantage of this decision and make some friends…. or maybe a little more.
(This is a Sombry-pie shipping, as I call it. Sombra will have small flashbacks to his past life. They will effect the future of the story. In the comments, you can put any ideas for new chapters or for Sombra's back-story.)