Age of Blood
When a player, NPC, or monster rolls a natural “20” on an attack it will be an automatic hit and double the damage dice should be rolled followed by adding the damage modifier one time per the PHB. In addition, the DM may determine a special effect appropriate to the specific situation such as disarming the opponent, knocking them prone, or forcing them to flee. The occurrence of and severity of said effect is at the discretion of the DM.
In the rare case that a player, NPC, or monster rolls two “20’s” on an attack roll when attacking with advantage or disadvantage the DM will determine a severe benefit appropriate to the specific situation up to and including breaking an enemy’s weapon, severing a limb, or an instant kill. This is in addition to the double damage described above.