Rare Musical Instrument (Double Bass), Requires Attunement

• As a bonus action, this big mechanical double bass can be transformed into a tiny pocket watch with the image of the bass on it. 
• Can be used as a spellcasting focus by bards or characters proficient in the Lyre, Lute, Dulcimer or Viol musical instruments. 

Killer Line. When you use this as a spellcasting focus, the bass creates a strong vibrating resonance and so each time you cast a spell of 1st level or higher, the target and all creatures within 10 feet of you must make a DC 14 Constitution saving throw, taking thunder damage equal to twice the level of the spell slot used on a failure. 

Power Chord. While attuned to the item, you can use your action to make a DC 14 (Lyre, Lute, Dulcimer or Viol) check. On a success, you cast the Thunderwave spell (DC 14 or your Bard spellcasting DC, whichever is higher) at 1st level as part of the action without expending any spell slots. If you succeed on the check by 6 or more, the spell is cast at 2nd level.

Curse of the Soloist (for GMs only). This musical instrument is semi-sentient and it absolutely hates incompetence and sharing the spotlight. Each time after you use it as a spellcasting focus to cast a spell on a different creature that benefits them and doesn’t cause direct harm. Or if you fail on the Power Chord instrument check, the string of the bass will start to move on its own and will try to restrain and strangle or garrotte you.
When that happens, you must make a DC 14 Strength saving throw, becoming restrained on a failure. While restrained in this way, you take 2d10 slashing damage at the start of each of your turns. You or a creature within 5 feet of you can use their action to make a DC 14 Strength ability check, ending the restrained condition on a success.