Life of the Party

From the introduction:

The characters are hired by Gisbert Handel, a local guildmaster and former apothecary, to provide security at a party he is hosting at his estate. When they arrive, they find Handel is the head of the new Assassin’s Guild, which is meeting with resistance from a number of prominent citizens. As the party begins, Handel has the doors locked and announces that as a showcase of his Guild’s skill, a pre-determined guest will be murdered by midnight. It is therefore the job of the player characters to prevent it from happening.