Travel to Draynor Manor and dig with a spade on top of the grave behind the manor to receive a skull.
Speak to Malignius Mortifer again. (2). He will ask you to collect 3 items which are picked randomly from a list of possible items. You can check the quest log to see what items you need.
After collecting the items, grab a pair of secateurs and an Amulet of Ghostspeak 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.