The Yewling and Aspen are an Arcane Operative team who lock down Heroes by entrapping them in a tree.
Yewlings used to be small forest tenders, guarded by the hulking Aspen warriors. The Darkspore changed them into dangerous abominations, using the trees the Aspens summoned to trap victims in life-sapping cages.
Yewling and Aspen combos are the Operatives for Arcanegenesis. The Yewling shows up first, staring at the the target. The stare is a debuff that targets the Hero for Lockdown. The Aspen then charges up and encases the target in a large tree.
By now the Yewling has created a shield around itself, making it much harder to kill. It tends the tree to keep the target locked, and the tree slowly saps the targets Health. During this the Aspen fights any interfering Heroes, hitting them with a strong melee attack.
The easiest way to defeat this team is to kill the Yewling before the Aspen comes, as the Aspen won't come if the Yewling is dead. However, the Yewling tries to stay hidden or behind other Enemies, making it hard to find and kill.