There are two types of Transmogs tied to the Nightfall Invasion, Arathi Armor Set and Nightfall Armor Set. Both sets have a transmog set each for plate, cloth, mail and leather and also different types of weapons. The Arathi Armor Set is the white set and the Nightfall Armor set is the purple set. These transmogs have different sources.
Arathi Armor Set

Sources
Armor
Source 1: Nightfall Event has a bar to fill. Each 33% of the bar rewards a bag (common, rare, epic), chance on Veteran Gear with the Arathi Armor transmog.
Source 2: Flame Radiance vendors sell Champion Gear with the Arathi Armor transmog. But, you only get one currency each week per account (not per character).
Weapon
Source 1: Nightfall Event has a bar to fill. Each 33% of the bar rewards a bag (common, rare, epic), chance on Veteran Gear with the Arathi Armor transmog.
Nightfall Armor Set

Sources
Armor
Source 1: Nightfall Event has a boss at the end of the event that rewards 100% loot. This loot is Veteran Gear, Nightfall Armor Transmog.
Source 2: Nightfall Invasion also consists of 3 daily quests. Each quest rewards a bag that contains high chance Veteran Gear, Nightfall Armor Transmog.
Weapon
Source 1: Nightfall Event has a boss at the end of the event that rewards 100% loot. This loot is Veteran Gear, Nightfall Armor Transmog.
Source 2: Nightfall Invasion also consists of 3 daily quests. Each quest rewards a bag that contains high chance Veteran Gear, Nightfall Armor Transmog.
Source 3: Flame Radiance vendors sell Champion Weapons, with the Nightfall Armor Transmog. But, you only get one currency each week per account (not per character).
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