Awakening of Evil

Listen, there's a non-functional Silvan tree in the center of the map (guarded by golden dragons).
It says it hasn't been visited - I click on it, and nothing happens.

The Silvan trees are scripted. When you click on one of them, the hero's level increases instantly without any further prompts. If one hero levels up at this tree, another hero won't be able to level up there anymore.