Thorian

The  Thorian, also called Species 37, is an ancient sentient plant at least fifty thousand 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 to control their behaviour. These 'thralls' then tend to the Thorian and obey its will.

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. Though it is intelligent, the Thorian only regards other sentient beings as potential thralls. It describes itself as "The Old Growth."

In Prothean Times
The Thorian existed on Feros long before the Protheans arrived. It would spend thousands of years in hibernation, then awake for "a few frantic centuries" of activity before resting again. When the Protheans arrived and began building the metropolis that would eventually cover Feros, the curious Thorian studied them, likely made several of them into its thralls, and even absorbed them after they died, allowing them to become a part of its expanding consciousness.

From the Protheans, the Thorian's mind gradually absorbed the knowledge that would form the Cipher. The Thorian survived the cataclysm that destroyed the Protheans, and continued to grow across Feros for the next fifty thousand years.

The Control Group
In the late 2170s, ExoGeni Corporation began settling Feros and established a colony in one of the ruined skyscrapers, not realising that the Thorian's lair was deep below it. The massive plant organism had awakened from its hibernation, and the presence of so many potential thralls so close was too tempting to resist. In 2183, a small survey team was exploring the ruins when they inadvertently entered the Thorian's lair and were infected with spores. ExoGeni realised what had happened, but instead of protecting their colonists they isolated the settlement to see what would happen and study the Thorian's massive sensory and mind-controlling potential.

As ExoGeni watched, it took less than a month for 80% of Zhu's Hope's inhabitants to become infected and begin performing tasks for the Thorian. The ExoGeni VI reports that via these spores, the Thorian issues mental commands to its thralls; if they resist, the thralls suffer intense pain until they are conditioned against any thoughts of rebellion. 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. The colonists at Zhu's Hope were therefore permitted to 'pantomime' a normal existence until the Thorian needed them, though some, like Ian Newstead, were strong enough to resist it for a time. The Thorian was careful to keep these few out of sight.

Mass Effect 3
When the Reapers attacked Zhu's Hope, the lingering hive mind effects of the spores allowed them to coordinate effectively. The colonists and Shiala were able to hold against the invading forces until they fully evacuated the colony.