When rolling a nat 20 on an attack roll you can choose to roll another attack roll. This roll does not gain advantage or disadvantage nor does it benefit from modifiers that add dice to your roll (like bless or bardic inspiration).
If the result of that second roll is higher than the attacked creatures ac you dubble the dmg of the nat 20's critical hit. However if the result of the second roll does not beat the attacked creatures AC the entire attack misses instead.
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