In sessions 7 and 8 the characters continued their investigation, shutting down the assassin training facility with the help of the city guard and eventually making their way into the sewers of Redboots, where they faced Rolovincio, down one of the leaders of the cult, and captured him. During his questioning they discovered the identities of all the plotters, and managed through deduction to figure out exactly where they all were. However, time was too short for them to make an effort to assault the keep where Zuthelia (the leader of the entire ring) was camped out, so they opted to set up during the Day of the King's Ear to prevent the assassination when she and/or her agents showed themselves. During their planning and investigations, it came to the attention of Prince Cassio that Decimus is an agent for the Aquilonian throne, working for King Conan II.
While they planned, it came to their attention that Rolovincio had tried to escape and been killed...when this happened he changed form to resemble a humanoid serpent. Merhotep and Valder, comparing notes from history and lore, came up with information about the Serpent Men, how they once ruled the world, how they were supposed to be extinct, and most importantly, they recalled the one phrase of power that the Serpent Men could not speak: "Ka nama kaa lajerama."
They set up so that everyone who entered audience with King Milo would be forced to speak the phrase, couching it as an ancient oath of fealty.
Unfortunately for them, Zuthelia was a powerful sorceress and slipped past with an illusion that she spoke the words.
She attempted to assassinate the king with a vial containing a mystical plague, but the PCs leapt into action and though a couple of the king's royal guard died of the plague, the king himself was saved, Zuthelia and her minions were all killed, and the rest of the serpent men (presumably) driven from Argos.
The PCs were paid handsomely for their efforts, and King Milo wished Decimus to take a show of gratitude (and solidarity) back to Aquilonia for him.