Revelations - The Demon Slayer - Platform: Gameboy Color

Ending bonus
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Defeat Lucifer to successfully complete the game. You may now 
explore any area in the game freely. Additionally, Lucifer can 
be found again inside the Cave of Oasis. 

Varieal
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To get Varieal, complete the game then go to the Cave of Zord. 
Go the end of the cave and Varieal will join your party. 

Hidden cave
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After flying through the Petroglyph (it is shaped like a bird, 
and how you get to Luciferium), climb out of the arc and walk 
around in the mountains north west of the Petroglyph. You will 
enter a secret cave holding the best sword and armor for Kishe, 
and the only true sword and armor for Uranus. Also, you can find 
Baal, Lucifer's bodyguard after you defeat him, in the place where 
Aman, Dragon, and Kirin are resurrected by bringing their three 
bones.