On Teen Wolf Season 6 Episode 18, Lydia believes that Halwyn, the "dead" hellhound, isn't actually completely dead. After Malia renders her unconscious, Lydia is able to communicate with Halwyn via a vision, and that's how Lydia learns that he's still alive, but there's a bullet in his skull.
Scott, Liam, Theo, and Mason listen to a voicemail that was found on a phone near the bodies of the pack in the woods that was killed by the anuke-ite. They believe the person who left the voicemail could be the anuke-ite's other face.
Malia and Lydia decide to put Halwyn in an MRI machine, which will rip out the bullet. Normally Halwyn would be able to heal from this, but unfortunately, the bullet is coated with silver. Once the bullet is out, Halwyn will only be alive for minutes before dying of silver poisoning. They decide Halwyn would be okay with dying if it meant stopping the anuke-ite.
Mason and Theo tackle finding Aaron, hoping to keep him isolated and therefore unable to merge with the other face, while Liam and Scott try to identify the voice on the voicemail.
Liam realizes that the voice belongs to his biology teacher, Mrs. Finch. After failing to expose her as a supernatural creature, Scott plays her the voicemail. Mrs. Finch reveals that the bodies in the woods were her family, and she left that voicemail for her daughter, Quinn.
Halwyn reveals to Lydia and Malia that they can't kill the anuke-ite, but they can trap it. If the anuke-ite merges, then it will kill anyone who looks at it.
Quinn is the other face of the anuke-ite. When Theo fights Aaron, Quinn is able to feel Aaron's pain. The two faces can sense each other, which is how they are able to meet up in the library and merge.
Later, the now merged anuke-ite goes to the morgue, looking for Halwyn. Instead, it finds Gerard who is there to make an offer. Gerard knows the anuke-ite never wants to be caught again, and to do that, he has to kill all the supernatural creatures.
Gerard offers to help the anuke-ite if it kills Scott McCall and his entire pack.