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This tabard was originally given as a reward from the level 60 Blood Elf Paladin dungeon quest True Masters of the Light, which also used to reward the Summon Thalassian Charger spell. As of patch 4.0.3a, that quest chain was removed from the game and the tabard was added as a reward for the new level 50 quest Weapons of Darkness.

Keep in mind that if you change faction, this tabard changes into a guild tabard since there is no Alliance equivalent. However, if you race change into a Tauren, you will still be able to keep the tabard. Like all tabards, you can repurchase it at a tabard vendor if you somehow loose or accidentally destroy it.