In 2375, Kurros and the "Think Tank" hired Y'Sek and his partners to attack the USS Voyager so that they could obtain Seven of Nine. First, the Hazari attacked the Voyager, and then it went to the "Think Tank" for help in fending them off. When they did, they demanded Seven of Nine.
During the first encounter between the Voyager and the Hazari, Y'Sek demanded their surrender. He refused to identify who had paid them to capture the ship. Captain Kathryn Janeway offered to pay them more, but Y'Sek refused. During a second attack, Y'Sek was captured by the Voyager crew, who determined after examining his ship that Kurros had posed as a Malon to hire the Hazari.
Janeway persuaded Y'Sek to team up with the Voyager crew to "out think the Think Tank." He realized that many races would pay large bounties to get their hands on the members of the Think Tank. After Seven of Nine and Janeway forced the Think Tank vessel out of subspace, Y'Sek and his fleet attacked them. (VOY: "Think Tank")