Teen Wolf: Season 3, Episode 4

3 - 4: Unleashed

The body of the newest murder victim is found. The Alphas attack Derek, and Deucalion offers him a proposition: join the Alpha Pack by killing his own Betas and absorbing their power. Stiles, Lydia and Deaton figure out that the serial killer is a Darach, a dark or evil druid. Mr. Harris is murdered next, though he seemed to know the killer. Derek, aware Deucalion wants him to kill his pack, kicks Isaac out, so Isaac moves in with Scott.