Critical Hits & Critical Misses
Rolling 1 or 20 PHB p194 [–] Sometimes fate blesses or curses a combatant, causing the novice to hit and the veteran to miss.
If the d20 roll for an attack is a 20, the attack hits regardless of any modifiers or the target's AC. This is called a critical hit, which is explained later in this chapter.
If the d20 roll for an attack is a 1, the attack misses regardless of any modifiers or the target's AC. Unseen Attackers and Targets PHB p194 [–] Combatants often try to escape their foes' notice by hiding, casting the invisibility spell, or lurking in darkness.
When you make an attack and succeed with a natural 20 (the number on the die is 20), or if the result of your attack exceeds the target's AC by 10, you achieve a critical success (also known as a critical hit).
If you critically succeed at a Strike, your attack deals double damage. Other attacks, such as spell attack rolls and some uses of the Athletics skill, describe the specific effects that occur when their outcomes are critical successes.
Many times, it's important to determine not only if you succeed or fail, but also how spectacularly you succeed or fail. Exceptional results—either good or bad—can cause you to critically succeed at or critically fail a check.
You critically succeed at a check when the check's result meets or exceeds the DC by 10 or more. If the check is an attack roll, this is sometimes called a critical hit. You can also critically fail a check. The rules for critical failure—sometimes called a fumble—are the same as those for a critical success, but in the other direction: if you fail a check by 10 or more, that's a critical failure.
If you rolled a 20 on the die (a “natural 20”), your result is one degree of success better than it would be by numbers alone. If you roll a 1 on the d20 (a “natural 1”), your result is one degree worse. This means that a natural 20 usually results in a critical success and natural 1 usually results in a critical failure. However, if you were going up against a very high DC, you might get only a success with a natural 20, or even a failure if 20 plus your total modifier is 10 or more below the DC. Likewise, if your modifier for a statistic is so high that adding it to a 1 from your d20 roll exceeds the DC by 10 or more, you can succeed even if you roll a natural 1! If a feat, magic item, spell, or other effect does not list a critical success or critical failure, treat is as an ordinary success or failure instead.
Some other abilities can change the degree of success for rolls you get. When resolving the effect of an ability that changes your degree of success, always apply the adjustment from a natural 20 or natural 1 before anything else.