You wreathe the weapon used in the casting of this spell in a jagged, screeching blackness that rends foes asunder with devilish ease. For the duration, the weapon deals an additional 2d8 piercing damage plus 2d8 necrotic damage on a hit.

Each instance of damage is rolled separately, including the base damage of the weapon itself, as the spell creates small aftershocks; this means that any creature that is forced to make a Concentration check imposed by this spell must roll one separately for each damage instance, for a total of three checks.