Chromatic Blade

You create a magical blade of shimmering, weightless force that constantly changes hue. You may choose to wield the blade or give it to an ally. The blade inflicts 4d8 force damage. When the wielder rolls a 20, the blade inflicts an additional 2d8 damage. Then the wielder rolls another d20. If you roll a 20, you sever one of the target’s limbs.

At Higher Levels. When you cast this spell using a spell slot of 6th level or higher, the damage increases by 1d8 for each slot level above 5th.