Rogue Legacy 2

First you have to beat Prime Irad, which can only be done in NG 5 and above.

After you beat him/it head back to the first area (citadel agartha). Go to the tall room where there are lots of hanging lanterns at different heights and a chest at the top. The map should show it as a green "unique" room. Shoot your magic or skill at the wall on the left side, about halfway up. It should reveal a hidden passage. Go up to get to a secret door.

Climb to the top and enter the door. The red scar memory should be on the right side.

Next time you're at the docks, you can enter the scar by talking to the sleepy guy. All you have to do is beat the boss once (no medal required). If you're having trouble, adjust some house rules (they don't deactivate achievements)