From Solium Infernum Wiki
Many actions require a contested challenge roll to work. To resolve the challenge, the attacker and defender each roll 2d6 (add the numbers from rolling two six sided dice) and add their modifiers. You can use the following chart to determine the approximate odds of the attacker succeeding by subtracting the defender's modifier from the attacker's modifier to determine the attacker's relative advantage. If you are defending, your odds are the remainder (if the attacker has a 3% of success, you must have a 97% chance of defending).
Some events require a challenge roll where you have to beat a predetermined number. These challenges are resolved by rolling a single six sided die and adding any bonuses. If the total is equal to or greater than the challenge number, you succeed (typically that just means bad stuff does not happen). For example, the event Infernal Perfidy requires each legion to make a loyalty check of 7 or better. To resolve it, a legion rolls d6 and adds their loyalty. If a legion has a loyalty of at least 6, they will automatically succeed (because you can't roll less than 1 on a d6).