Yarrick Zan was a mindflayer assassin hired by the Black Lotus to harass, and hopefully kill, the Planeswalkers. He had enlisted the aid of a group of slaughterhouse workers who had been students and minions of Llamar Fenwick., offering them the promise of revenge against their mentor's killer. Their base was an abandoned slaughterhouse in Sigil. His first attempt was on the Bishop and Savrin at the opera, but luckily, due to a tip-off from Enial, the rest of the Party arrived in time to thwart the attempt. Yarrick continued to harry the party using mind-controlled puppets (by using his innate ability to implant mental suggestions).
In a bid to get the assassin off their backs, the Planeswalkers arranged a meeting with him in a Lower Ward Tavern, offering to outbid his employers and pay him to leave them alone. Yarrick attended the meeting, but only to put his minions in position, at which point they attacked.
After a battle that quickly escalated into a tavern brawl, the Planeswalkers put down Yarrick Zan and all the remaining slaughterhouse workers, putting an end to the assassin's plot. (10, 11, 12)