Divergent Fist/Black Flash

By striking with the users fist using a thin layer of mana, a second wave of mana is unleashed a few moments afterward, effectively creating two impacts with one punch. However, if [roll]1d20[/roll] is rolled and the result is a Natural 20 the two layers of mana will overlap, and mana will flash black, the damage roll will be multiplied by 2 times.

TO DAMAGE ROLL DIVERGENT FIST:
roll a d6, then roll a d4.
TO DAMAGE ROLL BLACK FLASH:
You should get a critical that gives you 2d6. Simply roll those 2d6 and move on.