Invisible
Classic fifth-edition (2014) SRD 5.1 rules. The complete rules for this entry are reproduced below for use at the table.
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An invisible creature is impossible to see without the aid of magic or a special sense. For the purpose of hiding, the creature is heavily obscured. The creature's location can be detected by any noise it makes or any tracks it leaves.
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Attack rolls against the creature have disadvantage, and the creature's attack rolls have advantage.