Sometimes, two supernatural powers clearly oppose one another.
If the normal systems for the powers fail to resolve this, such as when a mage attempts to observe an effect with his Mage Sight that another has placed under a protective veil, there is a Clash of Wills.
All characters using conflicting powers enter a contested roll-off, each using a pool of Gnosis + Arcanum. Other supernatural beings have Clash of Wills dice pools specified in their own rules. Ties reroll until one player has accrued more success than all others.
The effect invoked by that player’s character wins out and resolves as usual, while all others fail. Victory of one power in a clash does not mean the immediate cancellation of the others, save in cases where only one power can possibly endure.
If the winning spell in a Clash between two spells runs out of Duration before the other does, the losing spell will take effect when the winner is removed.
Mages may spend Willpower to bolster the contested roll, and are always aware when their spells are clashing. Spells with Advanced Durations and Potency are more enduring in a Clash.
Day-long spells add +1 die to the Clash roll, week-long spells add +2, month-long +3, and year-long and indefinite effects add +4.
Advanced Potency adds +1 die.