Counterspell

Counterspell is a level 7 mage spell that interrupts enemy spellcasting. When successful, the target may not use the interrupted spell school for 6 seconds. For example, counterspelling a priest's Heal will not only interrupt the heal in progress, but also prevent them from casting Holy spells for 6 seconds.

Tips and tactics
This is a very potent spell in the mage arsenal when fighting enemy casters. Mages should learn when to use it.

In PvE groups, use Counterspell to pull ranged casters into the group. Chasing down a creep that is casting from range can be risky if other creeps are in aggro range. Counterspell will cause it to run towards your group, where it can be more safely killed. The spell generates a high amount of threat, so a mage casting it may find themselves at the top of the mob's hate list.

Counterspell is a "twitch" spell, requiring good reaction time. Consider placing this spell on an easily accessible hotkey. One possibility is to use extra mouse buttons, such as the middle scroll wheel or thumb buttons if your mouse has them.

Shields that absorb damage such as priest's Power Word: Shield and the paladin's Blessing of Protection do not stop Counterspell. However, Invulnerability effects such as paladin's Divine Shield will make the target immune, wasting your Counterspell.

While Counterspell is almost useless against warriors and rogues, it can be used to bring them into combat and prevent rogues from using Stealth. Given most other instant spells have cooldowns, if the mage intend to use Polymorph which heals the opponent, casting Counterspell to start combat does not waste any cooldown.

This spell can also be used to bring a mob group together in order to cast AoE spells on them.

For more info on using interrupts, see Tips and tactics.

Helpful macros
This macro will make your Counterspell hotkey more responsive, by stopping any current spell casts in progress. Since Counterspell is not affected by the global cooldown, the macro will work at any time.

/stopcasting /cast Counterspell

Trivia

 * Counterspell is a rare Mage spell in Hearthstone. The flavor text reads: "What's the difference between a mage playing with Counterspell and a mage who isn't? The mage who isn't is getting Pyroblasted in the face."