After the quest, you can find him near the king's house. As gratitude, he will sell you potions and he is one of the few merchants who sells utility items. He sells the following four potions:
When he joins the party temporary, he has the following stats:
Myrdinn Emrys is in fact the Welsh name of Merlin Ambrosius, the legendary wizard in the Arthurian legend. The achievement Ambrosius is another hint of this druid being Merlin.