While attuned with the whistle you can use an action to cast one of the following spells: Inflict wounds, Fear. Casting  a spell with the whistle doesn't use a spell slot, you can cast a spell this way once a day, it recharge at dawn.

If a creature, not attuned, tries to play the whistle must roll a Wisdom saving throw DC 15 or takes 2d4 psychic damage.

Notes: by all classes