Teen Wolf, Season 5

[Stiles and Lydia are talking to Dr. Valack about The Dread Doctors book]

Dr. Valack:
I wrote it because no one believed me. Because no one listened. They're here, aren't they? In Beacon Hills?

Stiles:
What are they?

Dr. Valack:
Not entirely human-- at least, not anymore. They were scientists once. Scientists who worshiped the supernatural. Tesla said, "If you want to find the secrets of the universe, think in terms of energy, frequency, and vibration." They found their secrets in electromagnetic forces-- ways to prolong their lives, give them power, and, most importantly, making you forget you ever saw them.