Things We Never Expected: Alpha's Mate (Sterek)

Stiles finds out he's Derek's mate. When they do mate....... Well the title says it all. Boyd and Erica are still alive, Erica is in a coma. Scott is not an alpha and part of the pack. Jackson comes back. Ethan and Aidan stay. Isaac and Alison are not together.
Season three has already happened. Other creatures come. Sheriff Stilinski knows.