As a reaction, which you use when another creature that you can touch or see within 10 meters makes an attack roll, an ability check, or a saving throw, you can hinder their effort. The creature makes the roll with disadvantage. You can do so after the roll but before any effects of the roll occur.
If the roll already has advantage or disadvantage, you cannot apply another instance of advantage or disadvantage and you must use this before the roll to cancel out any advantage or disadvantage.
You can hinder one additional creature for each additional mana expended. The creatures must be within 5 meters of each other when you target them.