Counterspell

As a reaction you attempt to interrupt a creature in the process of casting a spell. If the creature is casting a spell of 1st level its spell fails and has no effect. If it is casting a spell of 2nd level or higher, make an ability check using your spellcasting ability. The DC equals to 10 + the spells level difference between your spell's level and the level of the spell being casted.

As an action you attempt to break concentration of a creature concentrating on a spell. If the creature is casting a spell of 1st level the concentration is successfully broken. If it is casting a spell of 2nd level or higher, make an ability check using your spellcasting ability. The DC equals to 10 + the spells level difference between your spell's level and the level of the spell being casted.

You can't use counterspell to counterspell.

At Higher Levels. When you cast this spell using a spell slot of 2nd level or higher, the interrupted spell has no effect if its level is less than or equal to the level of the spell slot you used.

Classes: Sorcerer, Warlock, Wizard

Subclasses: Arcane Trickster Rogue, Eldritch Knight Fighter, Redemption Paladin

Subclasses (legacy): Redemption (UA) Paladin

Backgrounds: Azorius Functionary

Source: PHB, page 228