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How do you unlock ????

In The Binding of Isaac, ??? is unlocked by unlocking Have a Heart (Challenge).

Gives the Maze effect for the remainder of the floor. The effect can stack with other curses. If no other curses are active for the floor, Curse of the Maze will appear under the level name when Tab is held. This effect can be negated by picking up Black Candle, but will not prevent future uses of the ??? pill from giving the Curse of the Maze effect. As the pill's name is "???", the same as what unidentified pills are referred to, it is easy to accidentally use it more than once per run. It is recommended to memorize its color upon first using it. Using more than one ??? pill on a floor will add more curses.