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Darkfallen are a unique kind of undead blood elf in service to the Scourge that were raised as undead following Illidan's failed assault on the Frozen Throne.
The Darkfallen is a race of vampiric undead blood elves, under the command of the Blood-Queen Lana'thel. They are a recent addition to the Scourge army, introduced after Arthas' second visit to Northrend.
They possess their own racial faction, the San'layn, which plays an important role in the Scourge army.
The greatest concentrations of darkfallen are located in the Temple City of En'kilah in northeastern Borean Tundra, and in the Crimson Hall inside Icecrown Citadel; these are also the only places where unnamed darkfallen appear.
In the aftermath of the Third War, the blood elves were cut off from their source of magic, the Sunwell. Desperate for aid, their prince Kael'thas Sunstrider pledged allegiance to Illidan Stormrage who granted them a new source of magic. Together with their new allies, the blood elves went to Northrend in an attempt to destroy the Lich King, seeking to avenge their fallen brethren.
The blood elves were defeated by Arthas, who raised the most powerful ones into his service as darkfallen of the San'layn.
Most of the surviving blood elves returned to Outland with Kael'thas, but a handful were lost and scattered in the frozen wastes of Northrend, where they were hunted down and slaughtered by Arthas. One of these survivors was Lana'thel. Although she wielded the draconic blade Quel'Delar, she was no match for the power of Frostmourne and was overwhelmed. The Lich King chose Lana'thel to serve him in undeath as the queen of the San'layn.
The darkfallen now oversee the Scourge's operations throughout Azeroth, with their masters residing in The Crimson Hall. However, during the raid on Icecrown Citadel, many major darkfallen were killed. It is unknown what happened to the rest, but two weren't present at the raid and therefore, they're survivors.
Darkfallen mostly retain the builds of their still-living kin. However, unlike blood elves, their flesh appears necrotic and, though universally pale, ranges from fleshy pinks and red to gray and light green, and they tend to have either white, blond or dark hair.
Their eyes have white, green, blue or black glows. Some Darkfallen have clawed fingers and ears more closely resembling those of a bat; in addition, a few have developed wings and horns.
The darkfallen are skilled in a variety of magic schools, often make use of vampiric skills that involve blood, and have even demonstrated some shape-shifting abilities. They are vampiric in nature, especially in the case of the Blood Princes, who are known to consume the flesh, blood, and souls of their victims.
Darkfallen, most of the times, dress in red, orange and black garments, similar to their living cousins, the blood elves. They have assimilated the Scourge shared culture, and contributed with their own, which can be seen in the decoration of the Crimson Hall, inside the Icecrown Citadel.
They are often found in strongholds of other Scourge-affiliated races alongside with its local leadership, or in command of Scourge bases or operations across Northrend.
The San'layn is the Darkfallen racial faction of the Scourge. They oversee the Scourge's operations throughout Azeroth, acting as agents, leaders, or ambassadors. The faction holds an important position amongst the Scourge offensive, with its members, most of the times, performing important tasks on the behalf of the Lich King.
They possess two main bases of operations across Northrend: the Temple City of En'kilah and the Crimson Hall. The Temple City of En'kilah serves as the command center of the Scourge in the Borean Tundra, with the necropolis Naxxanar floating above the city where Prince Valanar, the overlord of the operation, lives. The Crimson Hall is the headquarters and home of the San'layn, located inside the Icecrown Citadel, and is the place where the most powerful San'layn can be found.
Blood-Queen Lana'thel holds the leading position in the San'layn hierarchy. The Blood princes, especially the Blood Prince Council, are the second in the chain of command of the faction, often receiving direct orders from the Lich King himself.