Instructions
Advance through the second stage of the game normally until you get to a cutscene involving the Behemoth, an enormous bull monster.
Run away from the Behemoth, and you will come across a series of holes in the ground. Drop down the first hole, and you will enter the sewers beneath the castle. This is an alternative route.
Continue to the right, smashing candles along the way in search of a crucifix, until you reach the Stage 2 boss, a giant shape-changing skeleton. This is a simple fight -- just dodge and whip the boss to death.
Begin the alternate third stage and travel to the right until you reach a floating skull mini-boss. Whip each of the skull's three eyeballs to defeat this mini-boss.
Go right and ascend the first flight of stairs you come across after defeating the mini-boss. Continue smashing candles along the way until you find the crucifix.
Turn left at the top of the stairs and continue along until you reach another flight of stairs.
Climb the stairs, and you will see a skeleton snake to your right and a large green ball of vines dangling overhead. Hop up and use your crucifix to cut the vines holding up the green platform. The platform will drop down, giving you access to the upper platforms.
Kill the snake, jump to the upper levels and travel to the right. Do not fall. If you fall, return to the green platform and try again. At the end of a series of platforms, you will find a statue. Smash it, and a glowing icon will float in the air. Walk up to it, and the game will alert you that you have successfully unlocked "Symphony of the Night."
Save your game, return to the main menu and select "Original Game" to play "Symphony of the Night."