In Stage 2, while the minotaur is chasing Richter, immediately fall into the first hole in the ground. You will not die, but instead be taken into the sewers. Play the level out normally and defeat the boss.
Afterwards, you will be taken to Stage 3 (this is different from the normal Stage 3).
In Stage 3, when the path branches, take the upper route. Make sure you obtain the "axe" sub weapon sometime during thIS stage.
Eventually, you will meet a skeletal snake that is attached to a wall. Nearby is a vine-like cage; use the axe to cut it. It now serves as a platform for Richter to jump to the upper ledges. Continue along the upper ledges until you see a breakable tombstone.
Inside is the item that unlocks CASTLEVANIA: SYMPHONY OF THE NIGHT. Return to the main menu to play it.