Source: Tasha's Cauldron of Everything

Wondrous item, rare (requires attunement by a bard)

While holding this concertina, you gain a +2 bonus to the saving throw DC of your bard spells.

As an action, you can use the concertina to cast Otto's Irresistible Dance from the item. This property of the concertina can't be used again until the next dawn.