Although using the Konami Code in Castlevania: Harmony of Dissonance takes some leg work, it’s well worth it.
First, you need to beat the game. Once you’re done celebrating your heroic victory, you’ll want to reboot the game and enter the code when you see the Konami logo pop up. If done correctly, you’ll unlock the ability to play as the 8-bit version of Simon Belmont from the original Castlevania in the Boss Rush mode.
Spoiler alert: it makes playing the game much harder.