Source: Critical Role

Weapon (longbow), ??? (requires attunement)

The bow is very well-crafted with elvish designs. There are bits of mithral scrolling throughout the bow itself and platinum caps on the ends.

Once per long rest, the wielder can use a Storm Arrow, which splits the next arrow fired from the bow into three separate arrows. Each arrow that hits deals an extra 1d6 Lightning damage.

If a magical arrow is shot from the Longbow, the two additional arrows would not copy its effects. However, each arrow can trigger Hunter's Mark damage independently. For example, the Blazing Bowstring can be used to add 1d10 fire damage to the initial arrow, but it does not affect the two additional arrows.