Sting (Bite in Beast Form)

Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 5 (1d4 + 3) piercing damage, and the target must make on a DC 11 Constitution saving throw, taking 10 (3d6) poison damage on a failed save, or half as much damage on a successful one.

Invisibility

The imp magically turns invisible until it attacks, or until its concentration ends (as if concentrating on a spell). Any equipment the imp wears or carries is invisible with it.