Ammunition of Backshot

An ammunition of backshot is a magic weapon designed to give an opportunity to double an archers' damage. When this arrow is used to make an attack roll, it can make another attack after hitting or missing; the extra attack must be on at a target you are aware of.
If the first attack hits, the ammunition can be used to attack one other target within 10 feet cone pointed in the direction of the attacker from the arrow.
If the first attack misses you can make another attack roll on the same target.

Any effects from the bow are put on both attacks with this ammunition, but other magical effects are not. This means you do not have advantage on the second attack when under the effects of something like Foresight; sneak attack is not magical and would work.
Once an arrow of  backshot makes its extra attack and/or damage, it becomes a nonmagical broken arrow.

You can technically attack people around corners without moving.

This is an Ammunition, you can use any weapon that utilizes Ammunition.

Proficiency with a Dart allows you to add your proficiency bonus to the attack roll for any attack you make with it.

Notes: For the creature making the attack to be attuned to the bow they are using; Ammunition of Backshot does not require attunement., Damage, Combat, Finesse, Thrown, Vex