Requirements: You need an Onion (they grow North of Rimmington), an Eye of Newt (Sold at the Port Sarim Magic Shop for 3gp), and Burnt Meat (Just cook cooked meat).
The Dark Side-
To start Witch’s Potion speak to Hetty in Rimmington and ask for work. Tell her you would like to become one with your darker side and she will ask you for some ingredients. You should already have all but the rat’s tail, which you can get by killing any of the rats wandering around the outside of her house. You cannot get the tail before or after the miniquest. Once you have all the ingredients speak to her again and then drink from the cauldron to complete the miniquest!