This is a video game walkthrough guide for "Onimusha: Way of the Sword," explaining how to solve the "Mask Puzzle" encountered by the character Musashi in the Underground Laboratory during the "What Lurks in the Shadow" section. The guide matters to players because the puzzle offers no in-room clues, making it easy to get stuck without external guidance, and it leads to further challenges including a contraption puzzle and an optional boss fight.

The solution involves observing shadows cast by masks near the room's entrance, which reveal the correct order to shoot hanging mask copies: Light Blue, Orange, Yellow, then Purple, in quick succession. Completing this rewards players with the Unsettling Mask, needed to access a nearby contraption puzzle solved by pulling two separate levers. The guide also covers a subsequent pitfall trap leading to an optional fight against the boss Byakue – Hundred Defilements, advising players to clear surrounding enemies, use environmental objects for early damage, and rely on parrying, blocking and well-timed "Issen" counters to win.