Speak to Malignius Mortifer again and he will ask you to collect 3 items. They are picked randomly from a list of possible items so take note of your own.
After collecting the items, grab some secateurs and travel to the Mort Myre Swamp south of Canifis and talk to the Nature Spirit (Follow the west side of the swamp until you come across an island with a jump-able bridge).
Take a filled Druid pouch with you if you had to collect any edibles or the ghasts will turn them into Rotten food.
Travel south-west from the chicken statue until you reach the wheat fields. Travel south of the wheat fields to reach the Cosmic altar. On the west side of the room, squeeze through a gap through the wall.