Bring cadava berries to the Apothecary to create a cadava potion (a potion that makes it seem like the person that drinks it is dead). A cadava berry bush can be found near the entrance of Apothecary's house. Then bring this potion to the hanging heads in Zemouregal's study (ground floor[UK], north-west section inside of his castle) which requires a vial of orange mist to enter.