The Peninsular War
Critical Hits and Failures
When you score a critical hit, you get to roll extra dice for the attack's damage against the target. Roll all of the attack's damage dice twice and add them together. Then add any relevant modifiers as normal. For example, if you score a critical hit with a dagger, roll 2d4 for the damage, rather than 1d4, and then add your relevant ability modifier. If the attack involves other damage dice, such as from the rogue's Sneak Attack feature, you roll those dice twice as well.
When a 1 is rolled on attack rolls, the attack misses and potentially hits an adjacent target, a target in the line of fire, or even the person doing the attack, depending on the type of attack being made. Once the adjacent target is chosen by the GM, the advantage roll is compared to their AC, and if it hits, the new target takes damage.
When a 1 is rolled on skill checks, the skill check not only fails, but potentially backfires in a way specified by the GM. For example, a character who critically fails an Acrobatics check might fall on their leg and develop a limp, or someone who critically fails an Animal Handling check might accidentally enrage the animal they're attempting to tame.