Ostinato

Incorporeal Movement. The ostinato can move through other creatures and objects as if they were difficult terrain. It takes 5 (1d10) force damage if it ends its turn inside an object.

Invisibility. The ostinato is invisible.

Magic Resistance. The ostinato has advantage on saving throws against spells and other magical effects.

Multiattack. The ostinato makes two Cacophony Burst attacks.

Cacophony Burst. Melee or Ranged Spell Attack: +7 to hit, range 60 ft., one target. Hit: 16 (3d8 + 3) thunder damage.

Aural Symbiosis (Recharge 6). One Humanoid that the ostinato can see within 5 feet of it must succeed on a DC 13 Charisma saving throw or the ostinato merges with the target, becoming an enjoyable, repetitive tune in its host's mind. The ostinato can't be targeted by any attack, spell, or other effect, and it can't attack. The host retains control of its body and is aware of the ostinato's presence only as a melody, not as a living entity. The host no longer needs to eat or drink, gains the ostinato's Magic Resistance trait, and has advantage on Charisma checks. In addition, the host has disadvantage on Wisdom saving throws, and it can't maintain concentration on spells or other effects. At the end of each long rest, the host can make a DC 13 Wisdom (Insight) check, realizing that the music it hears comes from an external entity on a success.

Contagious Tune (1/Day). While merged with a Humanoid host, the ostinato fills the minds of nearby creatures with the same catchy tune playing in its host's mind. Each creature within 30 feet of the ostinato's host must make a DC 13 Charisma saving throw, taking 21 (6d6) psychic damage on a failed save, or half as much damage on a successful one. The ostinato then gains temporary hp equal to the single highest amount of psychic damage dealt. A deafened creature is immune to Contagious Tune.