Phylarch the Evergreen

From user Poggor on the post here:

To get this epic follower you'll need a Level 3 Lumber Mill. To get the Tier 3 blueprints for your garrison buildings you will have to complete Upgrading the Mill, which includes turning in 75 orders in the Lumber Mill and completing Reduction in Force and Legacy of the Ancients.
After this you'll be able to log Large Timbers. Logging them will have Phylarch to spawn 3 times before he "surrender" and join your side, becoming one of your followers.

He can be assigned to the Lumber Mill as worker since he has the garrison ability Evergreen and there he will give Subversive Infestation.

So far, Phylarch is the only follower you get through the normal questlines that can work at your Lumber Mill. Once you have completed the Barov quest, the Ancient Trees quest and the achievement, he'll start popping up as you cut down some trees (I've noticed that he spawned at Large ones easily, but not that easily at normal or small ones). You must defeat him 3 times before he surrenders and becomes your follower. He sells [item=117398] for 250x [item=114781] once you have completed his quick quest in the Everbloom.

Go to Bloodmane Valley in Spires of Arak, cut down Large Timbers on the hilltop.
Did go several places, but he didn't spawn.
When I managed to figure out where he spawns I got him three times and he became my follower!