When you beat the final boss, (Maelstrom), that means you beat the ENTIRE game, you unlock insane mode. You also get the Maelstrom Heart from beating the game. (I will talk about its use later)
To access insane mode, go to the chapter select after selecting the file, and select Woodlands. (After clicking a file you see a Purple Magical stone with the Corruption logo on it.)
When you are in Woodlands, go to Frogville, and you will see the Mapleburg and Druid Grove doors, and a big dark purple door saying:
Insane Mode: Game Completion + MAX level required.
When you go by the door it will say:
Warning: This is an extremely difficult mode of the game! Are you sure you want to enter? Recommended for exerienced players!
If no, you don't enter and you are still in normal mode.
If yes, you are transported to Mapleburg, and you have started Insane Mode!
Insane mode is where ALL of the enemies (Even Bosses) are C3, when you fight a boss, they send three of their strongest minions. Example: BK sends out 3 burgulars, and BR sends out 3 golems. Also, battles are garunteed to have four people in them each. There are no dungeons in insane mode. If you do certain side quests or certain stuff, you can get items in insane mode you cannot get in normal mode.
If you manage to BEAT insane mode (Like you did in normal mode), then you get a dark purple block darker than the maelstrom heart, get sent back to the title screen, pick the file and it will show the Block AND the Maelstrom heart. Go to the Woodlands again, but go to your allies who know about the Block and the Heart, and they will congratulate you for beating insane mode and they have corrupt paste and you give them the two items, and then they turn the items into a Maelstrom Crown.
Maelstrom Crown: The crown for a true warrior of corruption normal mode and insane mode. When worn, it gives +32 POW and +32 DEF, Can't be sold. This is the reward for completing insane mode.