To start the quest, talk to Hetty who is in Rimmington which is South-West Falador.
Hetty: Greetings traveller
Hetty: What could you want with an old woman like me?
Player: I'm in search of a quest
Hetty: Hmm maybe i can think of something for you
Hetty: Would you like to become more proficient in the dark arts?
Player: Yes help me become one with my darker side
Hetty: Ok i'm going to make a potion to help bring out your darker self
Hetty: So that you can perform acts of dark magic with greater ease
Hetty: You will need certain ingredients
Player: What do i need
Hetty: You need an eye of newt, a rat's tail, an onion and a piece of burnt meat.
For the eye of newt: Go to Port Sarim and there will be a shop there owned by Betty who sells eyes of newts for 3gp each.
For the rat's tail: Kill any rat - a level 2 one will do - and they drop a rat's tail.
For the onion: There is an onion and cabbage patch north of Rimmington. Collect an onion from there.
For the piece of burnt meat: Keep cooking raw chicken/beef until you accidentally burn the meat.
Hetty: Greetings traveller
Hetty: So have youu found the things for the potion
Player: Yes i have everthing
Hetty: Excellent, can i have them then?
* You pass the ingredients to Hetty *
* Hetty put's all the ingredients in her cauldron *
* Hetty closes her eyes and begins to chant *
Hetty: Ok drink from the cauldron
Right click the cauldron and drink from
* You drink from the cauldron *
* You feel yourself imbued with power *
Well done you have completed the witches potion quest
Rewards: 3 magic levels and 1 quest point