Counterspell

Level 3 Abjuration spell available to Sorcerers, Warlocks, and Wizards. A Reaction that cancels an enemy spell as it is being cast. It works automatically on spells of Level 3 or lower, and needs an ability check for anything higher. It is the best defensive Reaction in the game and it is not close.

Spell description: Try to stop a spell being cast. If it is higher level than the spell slot you used to Counterspell, you must make a check using your spellcasting ability to prevent it.

Spell properties

• Range: 18m (60ft)
• Conditions applied: The targeted spell fails. Can be cast while Silenced.
• Cost: Reaction + Spell Slot
• Recharge: Per turn

Spell upcasting

Casting Counterspell at a higher level automatically counters any spell of that level or lower without requiring an ability check.

How to obtain Counterspell

• Sorcerer, Warlock, Wizard at Class level 5
• Eldritch Knight (Fighter) and Arcane Trickster (Rogue) at Class level 13

Source items: Staff of Interruption, Scroll of Counterspell

When to cast Counterspell

Cast it on the spell that would have ruined your turn: the enemy Fireball, the Hold Person on your tank, the Banishment, the heal on the boss. One slot to delete their whole Action is a trade you take every time.

It only fires if you have a Reaction left and a slot to spend, so on a Wizard it is worth leaving one Level 3 slot untouched all fight. Turn the Reaction prompt on for it, or the game will not ask.

Against a Level 4 or higher spell you roll a check, so the free win is not guaranteed. If you have the slot to spare, upcast it and skip the dice. A Warlock casting it from a Level 5 Pact slot counters almost anything automatically.

It works while Silenced, which the description mentions for a reason. The classic counter to a caster is Silence, and Counterspell is the one spell that shrugs it off.