If you obtain the eight Mystery Items, and use the Power of Compassion to end the eight dungeon boss battles peacefully, get the Power of Truth by collecting all the Corrupted Memories, and say Yes when asked to blow the dust out of the cartridge, you can visit Level 9.
Explore Level 9
If you have fulfilled the requirements listed above, then after the Delvin battle, you can choose the glitchy "Level 9" from the level select menu. If you selected a different level at first, you can go to the Key Items menu and use the Game Cartridge in the middle, then choose Level 9.
After some dialogue, go to the right, kill the monsters if you wish, then go up. Kill the skeletons in this room to get the blue key, then go back the way you came, unlock the blue door, and go through.
After you kill the monsters and do the block puzzle, go up. Kill the slimes and get the green key. Go back the way you came and go through the green door. Kill the monsters if you wish, and go up through the door.
From the classroom, go up to Lenna's room. After some dialogue, things will go black and white. Try to leave, and a battle will start.
You will soon be surrounded by glitch tiles. Knock the moving tiles back to avoid damage, then just wait for the background to go black. Walk off the edge of the screen. You can talk to the Elder if you wish. Check on the sword to get a Stone. Then the Elder will start shooting flames everywhere. You can be completely safe if you have the Fire Resistance Tunic equipped, or use Fire Resistance Potions. Once you hit the Elder enough times, you can use the Power of Compassion, or just hit him to proceed to the next phase of the battle.
Multiple Lennas will start walking around and attacking with the same weapons you have equipped. Go into the menu and change your weapon to "none", equip non-damaging items in all action buttons, or unequip your action button items by selecting the same items that you have equipped, then press the cancel button. The Lennas will die.
Then you will be outside of the Elder's Cave, and after a moment, Lance will come out of the cave and start attacking you. If you die, you'll just reappear at the cave entrance. Just hit him until you can use the Power of Compassion, or hit him a final time to end the battle.
Next, a face made of a couple of action buttons and some hearts will appear, surrounded by dungeon tiles. Jump over the dungeon tiles that fly off of it, and just wait for all of its surrounding dungeon tiles to disappear. It will turn into Archangel Lenna.
In this battle, you have to make Lance die. He will get hurt if he touches Lenna, or you can make Lenna throw her scythe downward by using a weapon. Once Lance dies, walk off the screen.
Then walk up and check on 8Fridge. Congratulations, you won! The ending will change depending on whether or not you unlocked multiplayer in the Shadow Temple, and whether or not you used the Power of Compassion on the Chairman, Delvin, the Elder, and Lance.