Realm of Toril
On a natural attack roll of 20 (so before any modifier is added or subtracted) an attack “critically strikes”, automatically hitting the target and dealing the maximum damage possible by the weapon used.
In addition to this, you may roll 1D20 again to “confirm” the critical strike. If you would hit the opponent again with this roll, you deal half again as much damage, rounded up. For example, if your max damage is 3, half would be considered 2 for a total of 5 points of damage on a confirmed critical strike.
If when confirming your critical strike, you roll another critical strike, your attack hits a truly vital point on the opponent, and slays them instantly no matter how many hit points they have remaining. Only the most powerful of enemies (Gods, Devils, and some other extraplanar creatures) are immune to this powerful and rare effect.