The Core Rule
The twenty-sided die (d20) is at the heart of Level Up’s action resolution mechanic. To make an attack, ability check, , or , players roll a d20 and add their relevant modifiers. Apply any bonuses and penalties, and compare the total to the appropriate difficulty rating (Armor Class, Difficulty Class, and so on). If the total equals or exceeds the rating, the roll is a success. Otherwise, it’s a failure.
Sometimes the Narrator will call for a group check. Group checks take place when the entire party is engaged in a single task, such as climbing a cliff or sneaking up on an enemy camp. The more skilled members of the group are able to help the less skilled members.
In a group check, every player makes an ability check or a . If more than half of the group succeeds in their check, the group as a whole succeeds. If half or less of the group succeed, the group as a whole fails.
Criticals and Fumbles
When you roll a 20 on a d20 attack roll, saving throw, or ability check (before applying any modifiers, bonuses, and penalties), this is referred to as a “natural 20”. Rolling a 1 on a d20 attack roll, saving throw, or ability check (before applying any modifiers, bonuses, and penalties) is referred to as a “natural 1”.
Criticals and fumbles also provide opportunities for additional positive and negative consequences. For example, in addition to being a guaranteed success, a critical hit increases the damage caused by an attack (dealing double damage). At the Narrator’s discretion, a fumble might also result in a complication for the attacker, such as their weapon getting stuck in a wooden door, requiring an ability check to free it.
Advantage, Disadvantage, and Expertise
There are two ways that a creature's chances of success can be increased or decreased.
Advantage & Disadvantage
Sometimes a circumstance grants advantage or disadvantage on an ability check, a , or an attack roll. When that happens, you roll two twenty-sided dice instead of one. For advantage, you use the higher of the two rolls. For disadvantage, you use the lower of the two rolls.
When you have both advantage and disadvantage (regardless of from how many sources), they cancel each other out and you roll normally.
Some class features or traits grant you an expertise die for an attack roll or , or in a specific skill or tool proficiency. When you make a d20 roll with which you have gained an expertise die, roll 1d4 and add the number rolled to the result of your check.
You can never roll more than one expertise die on the same roll. If another class feature or situation grants an expertise die that applies to the same roll, you don’t gain another die; instead, the size of the expertise die increases for that check, from 1d4 to 1d6, or 1d6 to 1d8. If you have a 1d8 expertise die on a check, further expertise dice have no effect.
If you have advantage or disadvantage at the same time as an expertise die, only the d20 is rolled twice, not the expertise die.
While advantage is most commonly used to represent circumstantial factors affecting a situation, expertise dice typically represent the particular training a character takes into the adventure.
Unless noted otherwise, when rounding in Level Up you round down.