In the fictional universe of Magic: The Gathering, Gix was a figure who lived during the time of the Thran empire. With the Thran-Phyrexian War, Gix joined Yawgmoth's side because he wanted to survive no matter what. A fictional universe is a cohesive imaginary world that serves as the setting or backdrop for one or (more commonly) multiple works of fiction. ... Magic: The Gathering (colloq. ...

Considering that Commander Gix wanted to survive, he made a good choice. The Thran of Dominaria have all been eradicated by Yawgmoth's bombs. The Phyrexians had won the war but at great costs. Yawgmoth became the ruler of the plane Phyrexia. It contained nine spheres—one inside the next, and descending down to the deeper spheres, they became more and more deadly. Having an important role in the war, Yawgmoth later promoted Gix to praetor—a Phyrexian with the power of life and death over anyone lower in the Phyrexian hierarchy than they are.

Gix became a member of the Phyrexian Inner Circle, a handful of the oldest, most compleat, and most powerful Phyrexians, some reaching as far back as the Thran Civil War. The Inner Circle had the closest contact to Yawgmoth and are ultimately responsible for carrying out his agenda.

When the Phyrexians became aware the ancient Thran lock at Koilos had been broken, Gix was chosen for an extremely important mission: see that one of the two brothers who fought for dominance of Dominaria become assimilated and then let the two brothers destroy themselves, leaving no resistance for a Phyrexian invasion of their homeland. Gix was one of the most powerful Phyrexians—but still not near as powerful as Yawgmoth. Gix got a group of humans who worshipped Phyrexia to work as spies monitoring Urza, Mishra, and any other power.

On Argoth, Gix had made all the artifact creatures of Urza and Mishra's war go berserk. Gix attempted to kill Tawnos but Ashnod died saving Tawnos's life. The huge sylex explosion had done minor damage to Gix as he had been transported back to Phyrexia, returning as a failure.

Yawgmoth banished Gix to the perilous Seventh Sphere. Barely surviving, itself a remarkable feat, Gix was redeemed.

Gix oversaw a new attempt of taking back his homeland through the sleeper agent project. Xantcha was one of the sleeper agents—Phyrexians in human flesh, unaware of being in total control by Phyrexia—but she had a great flaw of being self-aware. Sentenced to death, she was saved by Urza, the human turned planeswalker from over fifteen centuries past.

Preparing for the second invasion attempt, Gix was discovered again on Dominaria. Xantcha had seen him and told Urza, her new master. Gix traveled to Koilos and Urza followed after him.

Gix and Urza fought to the death, and Gix was gaining the upperhand. Xantcha and a human named Ratepe sacrificed themselves to disrupt Gix's power enough to let Urza finally kill the ancient Yawgmoth praetor." from http://www.phyrexia.com/continuity/Gix.shtml

Gix began as a humble peasant, one of the diseased untouchables that lived beneath the Thran capital city in a pit called Pyrexia. He plays an important role in the creation of the Pyrexian world, the destruction of Mishra, and Ursa's transcendence to a Planeswalker. Yawgamoth, the overlord of Phyrexia was once a human doctor. He was called to the Thran capital city to help a man that Gix had stabbed. Yawgamoths advances in medicine and technology eventually lead to the creation of the first phyrexians and eventually Phyrexia.

[1]phyrexia.com