In the mirror universe, Si Cwan was the Prince of the Thallonian Empire and older brother of Kalinda Si Cwan. He was the captain of the Thallonian flagship, the Stinger, which at its core, was powered by the god-like abilities of Mark McHenry.
His senior crew consisted of First Officer Zak Kebron, his oldest and most loyal friend, Head of Security Burgoyne 182 and his sister Kalinda, who was the Chief Engineer due to a strange gift she possessed that enabled her to connect with, and therefore control, McHenry. Cwan was also friends with Romulan Praetor Hiren, though in secret he and Kebron plotted to kill him.
In 2372, Cwan came to Romulus to meet with praetor Hiren who summoned him. After the early pleasantry, Hiren informed him that he believes that the Danteri were secretly building metaweapons and pose a threat to the Romulan Star Empire. Hiren asked Cwan to go to Danter and "provoke" them into using the weapons, which would prove the praetor right. in addition he asked Cwan to bring along a Romulan Businessman named Rojan, who, officially, would observe his meeting with the Danteri, but in truth, Hiren did not trust Rojan as well and wanted him "taken care of." Although Cwan at first questioned the praetor's information, he agreed for an amount of gold-pressed latinum.
When Cwan returned to the Stinger, he privately discussed with Kebron about his meeting with the Praetor. They both came to the conclusion that he was both paranoid and delusional, though Cwan made it clear that they were doing this mission in order to strengthen their bond with him for their plan to kill him.
Later, in the senior dining hall, Cwan had a welcoming dinner for his guests. during the meal, he found the treatment that Muck received from Rojan and his daughter alarming. He ordered his Terran slaves, Elizabeth Shelby and Robin Lefler, to come to him, and he warned them not to get any ideas. When he asked if they understood, they didn't reply and he struck Shelby at the side of her head and she fell. When Cwan observed Lefler taking a few steps back, he simply chuckled in amusement. He then asked Shelby again and she replied with yes, after this Cwan ordered her to get back to work.
Eventually the Stinger reached Danter and Cwan set up a meeting with Falkar, along with Rojan, in a conference room along side the bridge. when the meeting started, Cwan immediately declared that the praetor is convinced that Falkar and his people are constructing powerful Metaweapons to be used against the Romulan Empire and he demanded that they surrender and deliver all their weapon material. Before Falkar could truly object to this, Cwan Pulled out a Disruptor and shot him through the chest, killing him. Rojan demanded that Cwan explain himself, Cwan aimed his Disruptor at him and shot him through the head, killing him as well.
After this, Cwan, through the Loudspeaker, informed his crew that Falkar had attempted to assassinate both him and Rojan, and although he was able to kill Falkar in self-defense, he couldn't save Rojan.
Cwan then went to the bridge and began to target major cities on the planet's surface, when suddenly all the ship's functions went off line. When Cwan contacted Kalinda down in the Engineering room, it was Soleta who responded, for Muck was able to kill Burgoyne who was chasing them and nearly kill Kalinda, while Soleta was able to communicate with McHenry and awaken him. Soleta then demonstrated to Cwan their full control of the Stinger, and threatened him and his crew with the ship's interior security system.
Soleta demanded to Cwan that if he wished his sister to live, he and his crew are to leave the ship and then head for Romulus where he then must assassinate praetor Hiren. Though Kebron suggested to him that he can kill them, Cwan didn't want to endanger kalinda, and ordered his men to abandon ship.
Before anyone could leave, one of Cwan's crew members, believing that he is a cowered for doing this, shot at him with a Disruptor. However, Kebron was able to protect Cwan from the blast and crush the crew member to death. The Stinger's crew evacuated to the shuttlecrafts, and Cwan was the last one to leave the bridge. He swore that he will get revenge on the Romulan woman that turned on him.
However, the moment the shuttlecrafts were in space, Muck ordered McHenry to destroy them. The Stinger's guns shot at them and the shuttlecrafts exploded, killing Cwan. (NF - Mirror Universe novel: Cutting Ties)