Lydia figures out what Parrish is, thanks to Kira's story of The Wild Ride. Parrish is a hellhound. Right after she figures this out, she has a banshee vision - someone is going to die. Sadly, Theo shows up and tells her she can't tell anyone. He hits her.
Theo's not done yet. He traps Scott in the library. Theo surrounds the library with wolfsbane and then makes it so Scott can't call anyone.
That's right, Theo was able to spread wolfsbane. You know why? Because he is a chimera, the first one actually. He's part coyote.
Right after Scott realizes Theo gave him an inhaler laced with wolfsbane, Liam shows up to kick some ass. The super moon is fueling his anger towards Scott about Hayden. While Liam is trying to kill Scott, Hayden is dying. We're talking last couple minutes.
Theo tells Scott the truth about why he came for. He came for a pack, specifically Scott's pack, minus Scott. Stiles can either chose to save his dad or to save Scott.
Meanwhile, Braeden reunites with Malia and tells her that her mom knows she's still alive, and mommy dearest wants to kill her.
Mason, thankfully, shows up in time to snap Liam out of his rage. Hayden's dead. Just as Scott caught his breath, Theo shows up and kills him, but only for a bit. Melissa brings him back.
Theo uses Lydia to reveal the location of the nemeton, and once they are there, Theo brings back all of the dead chimeras by injecting them with something.