As a reaction, which you use when a creature that you can touch or see within 10 meters casts a spell, you can dampen the spell, making it easier to avoid or resist. The attack is made with disadvantage or each creature makes the first saving throw against the spell with advantage. 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.
Counterspell. You can expend 1 additional mana to attempt to interrupt the spell instead of dampen it. Make an ability check using your spellcasting skill. The Difficulty equals the creature’s spell Difficulty + the spell’s effective mana. If you have identified the spell being cast, you have advantage on this ability check.
On a success, the creature’s spell fails and has no effect.
On a failure, you dampen the spell as normal.
A spell’s effective mana is normally the amount of mana it was cast with, including augmentation. If a feature lets a spell be cast without expending mana then the spell’s effective mana is its normal mana cost.
You gain a +1 bonus to your ability check for each additional mana expended.