In Buffy: The Vampire Slayer season 7, the Scooby Gang faces off against the First Evil and its right-hand man, Caleb, a seemingly ordinary human with mysterious strength. The First Evil, a.k.a. just the First, originally appears as a monster of the week in Buffy: The Vampire Slayer season 3, episode 10, “Amends.” It manipulates Angel by taking the forms of people he killed, prompting him to attempt suicide, but he thankfully fails.
The First returns in Buffy: The Vampire Slayer season 7, the show’s final season. It’s trying to destroy the Slayer line after Buffy’s friends brought her back to life a season earlier, disrupting the natural order of things. The First takes the appearance of people who have died but doesn’t have a corporeal form. The Bringers—humans with their eyes cut out who are nonetheless capable fighters—do its dirty work, including killing Potential Slayers. Caleb (Nathan Fillion) is first introduced in Buffy: The Vampire Slayer season 7, episode 18, “Dirty Girls.” He’s a misogynistic ex-preacher serial killer working for the First Evil but hasn’t been cut up like the Bringers.