The Thorian, or Species 37, is an ancient, sapient species of plant. Its only known location is in the ExoGeni headquarters on Feros. Its most interesting attributes are its telepathic and mind-control abilities. It does this though a release of tiny spores; those who breathe them in become possessed and the Thorian is able to control them. The beings that they control are referred to as Thorian Thralls. It has the ability to generate copies of something that it has taken over, as shown in the game where it clones an asari commando several times. It can also create Thorian Creepers.
The Thorian is a very large organism, once covering much of the surface of Feros (Feros has a radius of 8366 kilometers). Its organization can be compared to one large neural system. There are several sensory nodes (some that are kilometers in length) that have smaller branches or tendrils. It uses the Thralls to tend to its large growth as well as protect it.
The Thorian is very intelligent. For unknown reasons, it allows its Thralls to continue living normal lives until they are needed. It also controls the Thralls through pain. It is, however, very careful to only hurt their puppets enough to force them into doing its bidding.
The Thorian is killed by Commander Shepard, who destroys its neural nodes to weaken its hold against the walls, causing it to fall and die.