Speak with Mathrengyl Bearwalker in the Cenarion Enclave, Darnassus.
Excuse me, friend...
It has come to my attention that Mathrengyl Bearwalker has put out word that he wishes to speak with you. It seems that your training as a druid is about to take on a new life, and he will be training you personally. Mathrengyl is the direct assistant to the Arch Druid himself, and is one of the most sage and august druids you'll find in Teldrassil.
He is located in the Cenarion Enclave of Darnassus, on the second floor of Fandral Staghelm's tree.