From The Abyss Codes -

Completion bonuses:
Once you have completed the game you will see an intermission sequence, and after that, the credits. You will be prompted to save the game. After saving, you will return to the title screen. Load your cleared saved game. Go into the shop for a new ring which adds + 15 to every stat. Note: You cannot buy it again if you sell it. Go into any abyss and fight. The Bosses are a lot stronger than they were previously.