Be attentive to all guests of OSRS GP the royal party. There will be King Vallance of Asgarnia looking ill. The slug will be the Black Knight Assasin entering the kitchen through the window. To find out if the fish is poisoned, examine it. Instead of throwing it away but instead, let Savant apply her magic to it. It will be served to King Vallance and he will go to sleep. Not dead, unconscious. King Vallance was paralyzed by the altered poison of the fish. Temple Knights have an excuse for taking him away to try and remove the slug.