How to Unlock Symphony of the Night

Progress through Stage 2 until you reach an area where a large horned monster chases you toward the right of the screen. While being chased, jump over the first two holes you encounter and fall into the third. After you fall into the underground passage, get the cross subweapon from a candle right before entering the boss chamber. Beat the boss at the end of the underground path to reach Stage 3' (note the apostrophe, rather than the regular Stage 3).

Head right through Stage 3'. If you lost the cross, you can obtain an axe at the start of Stage 3' by pushing the first tombstone you can touch, which drops you into a room beneath it that containing subweapons you can choose. Continuing along the path, you must defeat a three-eyed monster, jump across some platforms over quicksand, and run beneath a dinosaur skeleton. Next, you will see a staircase. Take the staircase up, then travel to the left through some obstacles and a few enemies. When you reach the next door, several staircases lead to an upper area. You will need a subweapon with good range, such as the axe or the cross.

Ascend the staircases into an area where you will immediately see a fire-breathing, skeletal snake and a platform entangled by vines. Kill the snake, then use a subweapon to break the vines from the hanging platform. For example, with the axe, stand beneath the platform and throw the axe up at the platform.

Use the platform to progress to the right, jumping across the upper path through the area. Before long, you will reach a tombstone at a dead-end in the top path. Strike the tombstone to break it, which reveals the "Symphony of the Night" item. Pick up the item, and "Symphony of the Night" is instantly unlocked and can be accessed from the title screen.