Thorian

"The Thorian is a piece of this world, extending across the land and back through the ages. You can no more kill it than you could cut the sky."

The Thorian, also called Species 37, is an ancient sentient plant at least 50,000 years old - its ability to hibernate for thousands of years makes its real age impossible to guess. The Thorian is a unique creature with mind-controlling and telepathic abilities, and a massive sensory network. It releases spores into the air that allow the Thorian to control those who inhale them, using pain as positive reinforcement. These 'thralls' then tend the Thorian and obey its will. Sometimes the Thorian spores can completely change a person's physiology, turning them into a mindless Thorian Creeper. It can even 'absorb' sentient creatures, creating clones of them which can speak for the Thorian when necessary. According to the VI at the ExoGeni Corporation Headquarters, the Thorian is essentially "a weave of tendrils" that covers much of the surface of Feros with some sensory tendrils kilometres long, though in places they are gathered into nerve bundles or neural nodes.

Commander Shepard first hears of the Thorian from Lizbeth Baynham, who tentatively suggests that it might be why the geth are attacking the colonists on Feros. The ExoGeni VI then explains more about it, describing the Zhu's Hope colony as a 'control group'. The Thorian is located beneath the settlement, further down the ruined skyscraper. When a few colonists were infected with spores by accident, ExoGeni isolated the settlement to see what would happen and study the Thorian's massive sensory and mind-controlling potential.

It took less than a month for 80% of Zhu's Hope's inhabitants to become infected and begin performing tasks for the Thorian. Once infected, the Thorian issues mental commands to its thralls; if they resist, the thrall suffers intense pain until they are conditioned against any thoughts of rebellion, which explains why the colonists at Zhu's Hope are acting so strangely. Though the initial conditioning is painful, the Thorian doesn't force the thralls to perform dangerous tasks or hurt themselves. The VI compares this behaviour to that of a craftsman who is careful not to damage his tools.

At first the colonists at Zhu's Hope are grateful and relieved to see the squad - the Thorian sees Shepard as the perfect weapon to destroy the geth who are killing its thralls. The Thorian instructs the salarian merchant Ledra to sell Shepard whatever stocks he has available, to help the commander out, and makes the other colonists urge Shepard to focus on fighting the geth. After Shepard goes to the ExoGeni building, the Thorian's massive sensory network likely picks up what is happening there. When the Thorian realises Shepard knows the truth about Zhu's Hope and is planning to kill it, the colonists turn on the squad and even try attacking the Normandy.

With help from Julianna Baynham, Shepard can convert the squad's grenades to explode in a cloud of gas that harmlessly paralyses the enthralled colonists. Shepard can also charge the colonists and knock them out with a melee attack-this is more difficult, however. The team fights their way back to the crashed freighter at Zhu's Hope, encountering Fai Dan on the way as the Thorian makes one last attempt to stop them. One of the cargo pods hides an underground entrance that the colonists were protecting.

Once underground Shepard and the team find a lair that smells like rotting compost. Down a set of stairs they are confronted with the Thorian itself... which is absolutely huge. The Thorian produces a clone of an asari named Shiala, who was absorbed by the plant on Saren's order as part of a trade. Through the clone, the Thorian, referring to itself as the 'Old Growth', commands Shepard to be in awe, saying every step the squad takes here is a transgression. It sees humans as "meat, good only to dig or decompose". After being betrayed by Saren, who sent the geth to destroy it once he had what he wanted, the Thorian has no interest in making a deal with "those who scurry" and begins sending clones and Thorian Creepers after the squad. Shepard destroys the neural nodes distributed through the building, dislodging the Thorian and killing it.

Once freed from the Thorian, Shiala then tells Shepard more about it. The Thorian was on Feros even before the Protheans arrived on the planet. It studied them and absorbed them after they died. This means that it was able to give Saren the Cipher after Shiala melded with it. Much to Shepard's surprise, Shiala is almost sad the Thorian is dead: despite what it did to the colonists and Shiala herself, it was a unique form of life that will never be seen again.