Supernatural is a multimedia franchise created by Eric Kripke. The series followed Dean and Sam as they track down their father. Along the way they run into just about every mythological and supernatural creature ever recorded. They usually kill them for the greater good.
As is usual with these kinds of shows, Supernatural has gone on long enough that Sam and Dean moved up to fighting against Hell and Lucifer, and have kinda killed God along the way too.
In Supernatural: Caledonia Sam and Dean travel to Scotland to meet up with an old friend of Sam's. Unfortunately the wind up in a city under seige by Selkie. The two eventually meet up with Sam's friend Emma, only for her to reveal that she, too, is one of the Selkie. Because she spent so much time on land, trying to be human, she influenced the other Selkie to stay on land as well, resulting in the attacks. The only way to save the town is for Emma to die.
Sam can not bring himself to kill her, however, so Emma ends up ending the threat with a bomb.