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A small black colored ball. Let's Isaac leave a trail of Black Creep behind him and ~5% of every shot has a chance of being a Tar Shot, which slows enemies down that are hit.
This item is particularly useful against enemies that relentlessly pursue Isaac, such as Globins, Lust, Gemini, Steven, and the Blighted Ovum, as the black creep left by Isaac renders them slower than Isaac himself.