The other girl who was out in the woods, Emily? Eventually, they're gonna find her. She's one of them. Emily, Heather, that guy Lydia found at the pool? All three were virgins. And they're all gonna have the same three injuries-- strangled, throat-slashed, head bashed in. It's called the three-fold death.
The Threefold Death Ritual, commonly referred to simply as the Threefold Death, was a ritual performed by the Darach, Jennifer Blake, in Season 3. Jennifer Blake used this ritual on many Beacon Hills citizens in an attempt to gain power in order to eventually fight the Alpha Pack. The ritual is performed by killing an individual three ways: a blow to the head; cutting of the throat; and strangulation by a garrote. Each sacrifice added to her power.
By sacrificing warriors she gained enhanced strength, Healers for healing abilities so great she healed Deucalion's blindness, Philosophers for intelligence and strategy, and Virgins, supposedly to seduce people easily for her goals, as with Derek. The Guardians sacrifice was possibly for protection.
The Sacrifices[]
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So, look, here's what I'm thinking-- I'm thinking that the murders maybe come in threes. Ancient people love things in threes, right? So, maybe first it's three virgins, and then, I don't know... Maybe it's three people who own little dogs?