Circumstance is a bonus or penalty to your roll based on temporary or situational things that either aid or hinder your attempt. If you’re attacking a target and another ally is flanking and distracting them this could grant you a Circumstantial Bonus. If you had sprained your wrist, and try to hit an enemy with a sword you could take a Circumstantial Penalty. Players are encouraged to ask about and suggest factors in the story which may give Circumstantial Bonuses and Penalties.
When performing a Skill Check if you roll either a 1, or a 20, on your 1d20, then your attempt either Critically Failed (in the case of a 1) or Critically Succeeded (in the case of a 20).
In the case of a Critical Failure treat your roll of 1 as if it were -10, and regardless of the total result, your action will go awry, or fail outright in some way.
In the case of a Critical Success, treat the 20 as a 30, and regardless of the total result your action will succeed in some spectacular way, at least partially.
After either of these take place the GM will put down a square of Empowered Ground nearby.
Once a Critical Success or Failure is rolled it cannot be changed. This means that you cannot reroll, or topple a dice that is resting on a 1 or a 20.