Sea of Eden
Introspection
Ness finds himself in a large body of water.
He is searching for something...
There are a handful of Kraken along the way. They can be troublesome if your level is too low so take precaution.
Ness finds... the stuff made of nightmares. After healing up, check it to start the battle.
Ness's Nightmare ( Battle )
Like most bosses, give him everything you've got as soon as possible.
Bags of Dragonite are great.
Keep an eye out for any shields that he might cast that would reflect your attacks back on you.
In general just keep at him until he's defeated. He will eventually run out of PP if the battle goes on long enough.
Sound Stone Surge
After Ness has defeated his nightmare he will hear his own voice telling him to go to Saturn Valley.
Suddenly, all of the power of the sanctuaries will surge through you causing you to level up like crazy!
You find ourself back at the Fire Spring where your friends are trying to wake you up.
The Sound Stone is gone.
You will now be automatically teleported to Saturn Valley.