Interact with and gain a War Guardian, and destroy 5 Giant Mushrooms.
The stone trogg mystics are creating those fungal monstrosities from the giant mushrooms to the north.
I want you to take a war guardian to help deal with the giants and stone troggs. Make your way to the giant mushrooms and target them. I and my geomancers will sink them into the ground before the mystics have a chance to finish their ritual.