Artereal Burst

You strike a target with necrotic power. Make a melee spell attack against the target. On a hit, the target takes 1d6 necrotic damage. If you have less than 20% of your maximum hit points remaining, when you hit with this attack you may treat it as if it was a critical hit.

This spell's damage increases by 1d6 when you reach 5th level (2d6), 11th level (3d6), and 17th level (4d6).