Fight for the Alliance

Log in during the two month international premiere of Warcraft the Movie.

Log in between May 25th and August 1st

Alliance get:
Fight for the Alliance
[item=140999]
[item=141000]

Horde get:
Fight for the Horde
[item=141001]
[item=141002]

Some classes are going to feel a bit neglected!

According to this official post from Vrakthris, getting the achievement will be enough to unlock the transmogs for use in the future Legion transmog system, so there is no need for license owners in trial to sub just to retrieve skins in their mailbox unless one wants them right now :

Getting the achievement just requires logging in, so while you won't be able to retrieve the items from the mail, that should otherwise qualify the account so when the transmog system is implemented they are automatically added to your collection.