Worshipper – “Conquest” [Campaign 2, Mission 3]

You don't necessarily have to kill the Druids. There are more than enough neutral creatures in the next mission to level up Agrael to the maximum.

I understand that we are talking about the source of druidic magic (or whatever it was called), and destroying it is one of the tasks of the mission. You need to destroy it at the beginning, so the elves won't have such a huge army.