Transform a minion into a 1/1 Sheep.
There was going to be a pun in this flavor text, but it just came out baa-d.
 How to get
Polymorph x 2 are automatically included in all players' collections.
- Polymorph is a permanent transformation effect that cannot be reversed; the targeted minion will remain as a sheep, even if it is returned to its owner's hand.
Polymorph is a powerful spell, especially considering the mage hero power allows the player to deal 1 damage to any target once per turn. This essentially allows a mage to destroy any targetable minion in play if they have a Polymorph spell and 6 mana to spare, making one of the most powerful removal options in the game.
Polymorph synergises well with 'control' decks.
Polymorph was first seen in Warcraft II as a mage spell which alters the physical form of its target. This turns any living creature into the critter for that map type such as Pig for Wasteland, Seal for Snow, Sheep for Forest, Boar for Swamp. Although the unit hasn't been killed it has been in effect killed as it cannot be turned back into the previous unit or commanded in any way.
In World of Warcraft, Polymorph is a mage crowd control spell which transforms the target into a harmless sheep. The spell has a number of differences from its incarnation in Hearthstone. Firstly, the effect is only temporary, lasting for up to 50 seconds against NPCs, or 8 seconds against players, and only one target can be polymorphed at a time. It is also broken by any damage taken by the target, and while the target is 'sheeped', quickly regenerates their health, swiftly negating any damage taken by the target before that point. Polymorphed targets are also unable to control their character in any way for the duration. These limitations make Polymorph great for crowd control - taking an additional target out of play while another is focused upon - but undesirable for use upon an active target.
While the sheep is the default form for Polymorph, various modifications exist to allow the mage to select other animals as the target form. Examples include rabbits, black cats, pigs, porcupines, bear cubs, penguins and more. When used upon critters, Glyph of Crittermorph increases the spell's duration to 24 hours and allows it to be used on multiple targets, allowing mages to transform whole zones full of critters into a variety of unlikely animals. Lakes full of aquatic bear cubs, and skies full of floating pigs, are just some of the possible uses of this glyph.
Mages have had Polymorph since the beginning of WoW. Originally, it was the most powered form of CC (crowd control), but has since been balanced by an abundance of CC by every class in modern WoW PvP. Before the addition of diminishing returns, mages were able to keep enemy NPCs and player characters polymorphed indefinitely.
- Ben Brode on Twitter. (2014-05-21).