They are the Sylvaneth. They speak for the trees.
Mostly because a lot of them are trees. Or, at least, they're partially-trees. You know, dryads and other things like that. And they're certainly marching to war with a whole bunch of new releases over in the Games Workshop webshop.
I did say there was a whole bunch of them. From Branchwyches to Kurnoth Hunters to Drycha Hamadreth to Alarielle the Everqueen. You've got the Battletome as well as a "how to paint" guide. There's even dice (because everyone loves having dice that match their army).
And tucked there in the corner is also the General's Handbook, which gives you new options for playing Age of Sigmar such as matched play and campaigns.