Bonus ending
After you've completed the game, let the credits pass by and you can continue playing. Just complete this part and you get to see the bonus ending.
Get the Feathers
Rahmu Feather
In the restaraunt there is a drunk marlboro in the top left corner. Keep on using thunder books on him until he turns back into Rahmu.
Sylph Feather
Go to the beach. When you see a treasure chest, take it and keep going back until you see a treasure again. Do this 3 times and Sylph will appear giving you the feather.
Titan Feather
Examine the statue in the center of town. After looking at it a few times you should see Ben there (a brown behemoth). He should ask you what the statue needs - if you answer by saying "an earthy smell" and come back
in a while he'll ask you for a quake book. Come back in a bit and the statue will turn into Titan.
Fight Black Goblin at the Beginning
When you get to the second floor of the first dungeon, you will see a goblin fighting
a muz. It will win and turn into a dark goblin. Let Mog get beat up by it and it will turn into a black goblin. You can try to fight it. You need to be at least level 15 or more to win.