From the Super Mario Wiki
The Mad Piano or Man-Eating Piano is a piano appearing in Super Mario 64 and Super Mario 64 DS. It looks normal at first, but when Luigi, Wario or Mario disturbs the instrument, it begins to open and close its lid, revealing sharp teeth. The Mad Piano then proceeds to chase and attack him (while making the sounds that sound like someone hitting its keys) until he has left the room. The Mad Piano appears only in Big Boo's Haunt. A Red Coin can be found behind it.
It is impossible to defeat this enemy; it can only be avoided.
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