Fight for the Horde

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

Log in between May 25th and August 1st

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

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

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.

If you logon during the during the two months following the international premiere of Warcraft (the movie), you'll get an achievement Fight for the Horde and an ingame mail from [npc=89753].

From: [npc=89753]
Subject: For the Horde!

We Orcs came to this world decades
ago, driven by demonic bloodlust.
Pillaging and carving a swath of
destruction. Now free of the blood
curse. We stand proudly alongside the
greatest warriors of azeroth.
Defending family and homeland with
our dying breath. But we must never
forget that dark past. Lest it overtake
us once again.Take these arms as a
reminder of our origins, the foe we
now face, and what it means to be of
Horde, Lok'tar Ogar!

The attachment in the mail is a [item=140998]. The box contains two items:
* [item=141001]
* [item=141002]