Spelltail Blowgun

Weapon (blowgun), Rare

This hollow bamboo rod is decorated with colorful feathers. Attack and damage rolls made with this weapon have a +1 bonus.

Tailing spell. When you hit a creature with a ranged weapon attack using this blowgun, you may cast a 1st-level spell or cantrip at the target as a free action. If the spell has an attack roll, use the same die result from the weapon attack for the first die. If the spell has a saving throw, use a 1 for the first die. The spell must have a casting time of an action or bonus action, target 1 creature (not self), and can't require a weapon.

("First die" means one of the dice if multiple are rolled, e.g. for advantage.)