Source: Dungeon Master's Guide

Wondrous item, rarity varies (requires attunement by a bard)

An instrument of the bards is an exquisite example of its kind, superior to an ordinary instrument in every way. Seven types of these instruments exist, each named after a legendary bard college. The following table lists the spells common to all instruments, as well as the spells specific to each one and its rarity. A creature that attempts to play the instrument without being attuned to it must succeed on a DC 15 Wisdom saving throw or take 2d4 psychic damage.

You can use an action to play the instrument and cast one of its spells. Once the instrument has been used to cast a spell, it can't be used to cast that spell again until the next dawn. The spells use your spellcasting ability and spell save DC.

You can play the instrument while casting a spell that causes any of its targets to be charmed on a failed saving throw, thereby imposing disadvantage on the save. This effect applies only if the spell has a somatic or a material component.

Instrument Rarity Spells
All - Fly, Invisibility, Levitate, Protection from Evil and Good, plus the spells listed for the particular instrument
Anstruth harp Very rare Control Weather, Cure Wounds (5th level), Wall of Thorns
Canaith mandolin Rare Cure Wounds (3rd level), Dispel Magic, Protection from Energy (lightning only)
Cli lyre Rare Stone Shape, Wall of Fire, Wind Wall
Doss lute Uncommon Animal Friendship, Protection from Energy (fire only), Protection from Poison
Fochlucan bandore Uncommon Entangle, Faerie Fire, Shillelagh, Speak with Animals
Mac-Fuirmidh cittern Uncommon Barkskin, Cure Wounds, Fog Cloud
Ollamh harp Legendary Confusion, Control Weather, Fire Storm