Whenever a Power or other effect says “Damage”, it always means “Damage to Invaders” unless explicitly specified otherwise.
When dealing Damage, it is done to any Invaders in the target land, divided among them however you choose.
If an Invader has taken Damage equal to or greater than its Health, it is Destroyed immediately.
Damage does not change the Health of a piece or how much Damage it will do during Ravage.
Damage may be combined from multiple Powers. However, any remaining Damage on survivors heals at the end of the turn during Time Passes.
Invaders are not the only recipients of Damage in Horizons of Spirit Island: the land and Dahan can take Damage from Invaders Ravaging.
But the overwhelming majority of Damage in the game is done to Invaders.
Any time a Power, Fear Card, or other game rule does Damage, it always means “Damage to Invaders” unless explicitly specified otherwise. Most Spirit Powers do not harm the land or Dahan.