Potestas
Magic allows characters to change themselves and the world around them, whether by force of will (Mysticism) or by tapping otherworldly power (Sorcery). Although the effects of a spell will vary within the game setting according to the type of magic used, the underlying game mechanics are the same for both.
To cast a spell, the player must make either a Sorcery or Mysticism Resolution Roll (a Risky Success is possible). All spells have a Difficulty modifier that alters the Target Number. If the caster succeeds, the spell is cast. If there is a Risky Success or Failure, the attempt automatically causes immediate Fatigue equal to the Degree of Failure.
Characters can cast up to three spells in a Round, but must divide the Sorcery or Mysticism bonus between each spell, increasing the chance of a Risky Success or Failure. You cannot cast three Sorcery and three Mysticism spells in a round if you have both Skills: it is a maximum of three spells.
Some spells directly affect the target (e.g. causing a disease, forcing the target to obey the caster), while others create a physical effect (e.g. fire, moving an object) that then affects the target. Spells that create a physical effect use the same game mechanics as non-magical physical effects (e.g. ranged attacks have a Speed modifier and can be dodged). Whenever a spell directly affects a character, they must make a Willpower Resolution Roll. The character adds their Stature to the roll and the spell's Power is added to the Target Number. If the character succeeds, they have resisted the enchantment.
Characters can increase the Power of a spell by +1 for every +2 Difficulty added to the Target Number for the Mysticism or Sorcery Resolution Roll.
Different magic spells last for different amounts of time. Some spells last forever once cast (e.g. Heal or Harm, Hurl Fire), others last until they are dispelled (e.g. Create Illusion; see also the description of Control Magic). Both of these types of magic are considered permanent. Impermanent magic lasts for only a few minutes at best: the player must make a Spell Expiry Roll when the character first has an opportunity to be the active character in each Round On a roll of 2 or 20, the spell lapses.