The mystics of the Elder Council set up two seal boundaries in the Tree Cave to prevent mortals and monsters from entering.
It appears that the seal stops people, but not monsters. Luckily, the monsters cannot break the second seal and get to the Tree Cave's roots.
We must enter the Tree Cave, slay the monsters, and retrieve their abnormal energy. We can use it to figure out a way to fix the boundaries.
Check out the drawings in Karda's journal - one of the monsters appears human but has insect legs growing out of its back. It exudes strong magic energy.
Karda's journal says that the creepy man-insect hybrid attacked them after they obtained the Lion's Sword. It's highly intelligent, hides behind other monsters, and directs its attacks.
Karda also felt a strongly oppressive, twisted magic emanating from the monster.
That particular monster must be their leader... or at least their advisor. Or, who knows, maybe a career counselor.