The climax of almost every "I did something awesome, guys!" story on /tg/. A natural 20 is when you roll 20 on a 20-sided die (most commonly used by Dungeons and Dragons). In almost every system this is a critical success, meaning that whatever you did happens without question, even if it should be impossible for you to do based on your skill scores or stats. As such, it's a convenient way for a GM (or an OP) who's completely telling the truth about how the story went to make it go the way he wants.
Crits in Other Games
The Natural 20 signifies the crit, as stated above, which causes inhuman and unnatural levels of goofiness and awesome. However, not all systems use the D20, so this handy list will try to encompass all of the Crit or Auto-Pass rolls for each system:
- Dark Heresy: A roll of 1 on a D100, called a "Natural 1". In combat the opposite applies, where rolling the max on a damage die (usually a d10) triggers a crit, prompting either extra damage or minor handicapping based on edition.
- Battletech: A ranged attack against a Battlemech that rolls a 12 with 2d6 on the hit location table yields a headshot, thus fucking up anyone with your Clan tech ER PPC.
- GURPS: a three or four on 3d6 for any success roll on which an attempt was even allowed, rolls of five or lower on rolls with an effective skill of 15+ and six or less on effective skill of 16+. The main effect of a critical hit in battle is that the attack was successful and the defender doesn't get to attempt an Active Defense to avoid it. A separate roll determines whether a critical hit does the attack's usual maximum damage, ignores some or all of the target's armor, does double damage, or does triple damage. The most common result is "no extra damage". Critical successes on non-combat skills generally have less detailed rules provided, as is to be expected.
- In Nomine: The only thing in the rule system that was both intentional and interesting, if only in a "I see what you did there" way, you rolled 3d6, and if you got a 111, the result was always tilted towards Heaven's interests (a "Divine Intervention"), and if you got a 666, the result was always tilted to Hell's interests (an "Infernal Intervention").
- Iron Kingdoms RPG: Crits in Iron Kingdoms RPG are weird. If you roll 2 or more d6's and you get two of the same number, you get a critical hit, as long as you hit your targets as well. Thing is, this generates a feat point and may have other benefits depending on what you just did, but it's not an "Automatic Hit" like a natural 20, as demonstrated through having to hit before anything fun will happen - Instead, a true Critical Hit happens when you roll two 6's on two or more dice, which of course means that the more dice you get to use in an attack, the more chance for mayhem. Also, this does the same as the "regular" crit described above. In Warmachine and Hordes, critical hits don't do anything special unless the model attacking has rules which say so.
- Pathfinder: Being based on the D20 system, a natural 20 is Pathfinder's Critical Hit indicator.
- Paranoia: Paranoia uses a d20 system where the objective is to roll low, so it's backwards from D&D; a 20 is a critical failure, and a 1 is a critical success. However, the true result of any roll in Paranoia will be either what Friend Computer considers to be just or what the GM considers to be funny.
- World of Darkness: When one dice in a pool of d10s rolls a 10 (or 9 or 8 depending on some specifics with the test), you can roll that dice again and potentially add it to your successes.
- Wrath & Glory: Rolling a 6 in a pool counts as two successes in a pool. If you have more successes than you need for a test, you can transfer any extra successes (usually by rolling a 6 alongside anything above a 3 on each die) to add extra benefits to the test, from expediency to extra damage.