Mirror Entity

Mirror Entity is a common mage spell card, inducted into the Legacy set, formerly from the Classic set.

Fixed bugs

 * Until Patch 7.1, Mirror Entity would copy a played minion even if it is mortally wounded by Snipe, already removed from play, including the interesting case case where it is mortally wounded, and a friendly Health Aura saves the copy from dying.

Strategy
This card is best played in the late game, as it has a higher chance to copy a strong minion then. It is also useful for countermanding an opponent with an empty board who may attempt to play in a large minion to regain board presence. In fact, a good strategy to coerce your opponent into giving you a free minion is to flood your side of the battlefield with smaller minions or a large one of you own, as this will likely prompt them to play something costly in order to defend themselves.

When playing against a Mage, this secret can be baited by playing a weak minion before playing any larger ones. If this secret does not trigger, you are safe to play larger minions; if the secret does trigger, it will only copy the weak minion, and you will then be free to play larger ones. Playing a weak minion also may allow you to attack with them, possibly dealing with Vaporize, another popular Mage secret. The effect of Mirror Entity can also be mitigated somewhat by playing an understatted minion with a powerful non-buffing Battlecry: the Battlecry will only activate for the original minion, and not for the copy. Bear in mind though that if the Battlecry buffs the summoned minion, Mirror Entity will copy the buff along with the minion, making cards like Twilight Drake most often a bad choice.

Lore
This spell doesn't seem to have a precise match in Warcraft lore. However, mages are well-known as masters of illusions, able to summon multiple fully-functioning images of themselves to assist in battle or even teaching and spell-casting, such as through the spell Mirror Image.

Trivia

 * This card's art is a reference to Valve's  game series. The games feature a "portal gun" capable of opening small wormholes between two locations through which the player and other objects can pass, with one portal appearing light blue and the other orange. The two modes of the portal gun also caused it to glow the colour of the corresponding portal, similar to the mage's hands in this card's art. The mage also has a necklace and a badge on his left arm resembling the Aperture Science Weighted Storage Cube and Aperture Science logo respectively. However, the portals only move objects around, they do not mirror or duplicate them.

Patch changes

 * Fixed a bug in Patch 20.4.0.84593 where Archdruid Naralex copied by Mirror Entity would give Dream cards to your opponent instead of you.
 * Moved from Classic to Legacy set.
 * Now reads "After your opponent" instead of "When your opponent".
 * Now activates when your opponent plays a minion, rather than when an enemy minion attacks.
 * Now activates when your opponent plays a minion, rather than when an enemy minion attacks.