Due to many people asking me where to buy pot I decided to make a guide on the event. There will be a map for everything at the end of the guide. Hope this helps.
(http://i.imgur.com/9XMdbtE.png)
First off you will need to talk to the grim reaper. He can be found sitting in his chair on the first floor of draynor manor. Once you talk to him, he complains about people only dressing up and eating candy and will ask you to teach them the art of trick or treating. He will tell you to visit a barbarian, a cook and a farmer.
(http://i.imgur.com/3pRXWCO.png)
The barbarian
He can be found guarding the draynor jail east of draynor. He doesn't get scared of you at all after talking to him the first time, so you'll have to go back to the grim reaper. Despite the barbarian not having learnt very much, the grim reaper is satisfied with your effort and you can move on to the next task.
(http://i.imgur.com/9DMzviM.png)
The cook
For this you will need bone munchies. They can be made by using a pot of bonemeal on a pot of flour. For this you will need two empty pots, which can be bought at the general store. You will also need a pestle and mortar, some bones and grain. Grain can be picked from any wheat field in emps scape. Use the pestle and mortar on the bones and grain and they'll turn into a pot of bonemeal and a pot of flour. Use these two pots on eachother and you will get the bone munchies.
Head to the cook in draynor with the munchies. He can be found in one of the houses. After giving the munchies to him he will notice it's a trick, he will still reward you with some food for your efforts though, you can choose between raw and cooked food. Which will either be 20 raw or 20 cooked swordfish. Return to the grim reaper for the next step.
(http://i.imgur.com/QT3zYXu.png)
The farmer
The grim reaper will tell you this farmer still owes him some pumpkins from last year's harvest. You will have to scare him with the pumpkin mask the grim reaper gives you. He can be found on his farm east of draynor. Wear the pumpkin mask and talk to him, he will tell you he will deliver the pumpkins on time next year. Return to the grim reaper for your reward.
(http://i.imgur.com/n0TYcnv.png)
Rewards
You will keep the pumpkin mask you get during the event as a reward and you will also get the skeleton ring. The skeleton ring will turn you into a skeleton, much like the perfect ring turns you into santa. It's wielded in the weapon slot and looks like this:
(http://i.imgur.com/FI7ScsO.png)
Map
(http://i.imgur.com/O3uFGvw.png)
Happy halloween!