The Magoi are a very frail race of abnormals that defend themselves by manipulating other beings' mind and having those beings kill each other and themselves. For example. a Magoi once tried to kill Henry by having him stay in a room that was slowly filling with water, making him think the door was locked when really it didn't even have a handle.
If you ever have to face a magoi try to keep your group as close together as possible because it'll be harder for the magoi to trick several people at once and keep everyone in the same place as the magoi could disguise itself as one of you. Look for subtle differences.
In "Kush", Magnus and Will traveled to the Hindu Kush in search of a Yeti. They believed they had succeeded in capturing one and loaded it on to a plane bound for the New Delhi Sanctuary. However, the creature they captured wasn't a Yeti, but in reality a magoi which had used its' psychic abilities to make them believe it was a Yeti. The magoi escaped by tricking Will into not locking its' cage and then killed the plane's pilot thereby causing a plane crash. The magoi soon claimed the plane as its' territory and began killing off the passengers. Magnus was finally able to kill the magoi when it disguised itself as Will and she asked it for coffee with it saying yes Magnus knew that it wasn't Will as the real Will knows she hates coffee.
About three years later, a group of Magoi were discovered frozen on a Russian icebreaker full of dead crew and abnormals. the two free ones that killed the crew pretended to be Helen and Will in order to manipulate the Sanctuary team, consisting of Henry Foss, Declan MacRae, Alistair, and two others in order to save the rest of them. They tricked them into believing that salt water supercharges Magoi, in order to get the team to move the frozen ones to higher ground in order to be saved from the rising salt water.
However, this plan fell apart when the real Magnus and Will arrived, and Magnus disproved her double's statement about the salt water by throwing herself and the Magoi into the water killing the creature. Will then shot and killed his double. The team took the rest of them to the Sanctuary, noting that they had not done anything wrong and did not have the blood of the Russian crew, or the three Sanctuary members on their hands.