Before the novel
Simenon helped Vilnix to make more chains for Sanctaphrax, making them to replace and add to the Anchor Chain. He enslaved Undertowners to make chains in the factories, and he polluted the Mire outside Undertown and made it even more unhospitable for life. Simenon was very corrupt, and he liked making friends in high places.
During the novel
Simenon Xintax was Slyvo Spleethe's boss, and Spleethe reported to him after a mission with Quintinus. Spleethe orchestrated his mutiny with Xintax and the Leagues all propogated questions at Spleethe about his honesty and desires. Simenon asked Spleethe to return with the stormphrax and give it to them. Spleethe asked if he could become Leaguesmaster himself and replace Simenon if he was successful. Xintax paused a bit and then said yes.
Xintax eventually dismissed Spleethe, not before warning him of the price to pay if he betrayed them.
Later, after Spleethe hadn't returned from his mission, Xintax assumed he had failed because Quintinius told Spleethe that Xintax wanted him to fail. Xintax gleefully set about business as usual, and he confessed to Polmpolnius himself about his dealings with Spleethe and the knowledge he had about matters before they happened. Unintentionally, Simenon Xintax was the cause of Vilnix's downfall, because Vilnix confessed to his manservant about his treachery, and this led to his manservant confiding in the professor of Darkness and exposing his master's evil.
The treacherous Leaguesmaster was eventually killed during a business meeting when Twig's ship smashed apart and the rudder wheel fell down on the League chamber, killing Simenon and his colleagues.