Find a pouch of bittersweet and a pouch of wild ginger, both dropped by Minotaur Shamans.
Buy a green forest spirit stone for 20,000 gold pieces from Elysanna Blackrose, located at Silversky, by saying green stone.
The green stone is needed to speak with Greyleaf, otherwise he refuses to talk to you.
Bring them to Greyleaf, located east of the Tarantula Lake, and say useful, then yes.
Reward: