There's a fairy nearby who talks about sensing the approach of a terrible evil.
This is the quest's super-boss.
To access it, you must complete two other tasks,
First is the mini-game replay, which requires you to finish the mini-games first.
Then, talk to Farore in Nayru's house and replay the mini-games at least once.
This will reward you with an item.
Next is the boss rush. This requires each character to access a secret area.
Din's is a warp point in the Frigid Fields that leads to Crescent Island.
Trigger an Ether switch there to access her side.
Nayru's is a warp point inside the former location of Moonlit Grotto.
Requires the Sword of Fate to enter.
Each character must then complete their boss rush and get the reward.
Once you have both, speak to that fairy in the underground village to face the super-boss.