Demon Worms dwelling in the Tree Cave caused the tree's illness. I believe they are giant ants with horns!
To heal Gaia's Tree, we must eliminate the monsters, then pour the healing Sacred Spring onto its roots.
To reach the roots deep in the Tree Cave, you must first pass a seal. You'll need the Lion's Sword to do so.
The Lion's Sword was left behind by the slain healing team. It should be near the monsters. Please get it back.
The Lion's Sword was created by the Elder Prophets to seal the Barrier Runestone in Gaia's Tree Cave. It is also the key to crossing the Seal Boundary.
The healing team met their demise soon after finding the sword. Monsters swarmed them. They never had a chance. Borg shuddered.
Brave one, after you enter the Tree Cave, be extremely careful. Those monsters will mash you the first chance they get.