To beat the game, you need to collect the masks of the three different monster types. You can then wear them with Z, X, or C. Wearing a mask gives you a bonus to hide from monsters of that type. You need them all to be able to get out of the final room.
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The first mask is in the third room of the game, below the right-most coffin. It looks like the vampires's head, but with black eyes. They were supposed to have a glowing effect, but that got axed for time. It seems like I should have probably made the time for adding that effect. Once you grab the mask, the Icon at the top left of the screen tells you which button you use to toggle it. To make it easier for WASD users, JKL will toggle masks just like ZXC.