The San'layn (sometimes spelled in lowercase, also known as the Darkfallen) are vampyr Blood Elves in service to the Scourge. They used to be among Kael'thas Sunstrider's greatest followers, until they were raised into undeath by Arthas following Illidan Stormrage's failed assault on the Frozen Throne. During the Blood War, a number of San'layn led by Blood Prince Dreven were given a chance to join the Horde by Warchief Sylvanas if they could work with them, but Dreven was killed during his mission.
The Third War
In the aftermath of the Third War, the Sin'dorei were cut off from their source of magic, the Sunwell. Desperate for aid, their prince Kael'thas Sunstrider pledged allegiance to Illidan Stormrage who taught them how to siphon magic, and how to wield fel. The blood elves, or Sunfury, seeking to avenge their fallen brethren, participated in Illidan's attempt to destroy the Lich King. Arthas intervened, and the two armies clashed, but the undead ultimately defeated Illidan's forces at the foot of Icecrown. The surviving blood elves returned to Outland along with Kael'thas.
A few remained lost and scattered in the frozen wastes of Northrend, but Arthas sought out one by one those who had challenged the Lich King. He shattered their forces and raised the most powerful ones into the Lich King' service as San'layn. Among those survivors was Lana'thel, wielder of the draconic blade Quel'Delar, who was no match for the power of Frostmourne. She was chosen by the Lich King to serve him in undeath as the Queen of the San'layn.
War against the Lich King
The San'layn oversaw the Scourge's operations throughout Azeroth, holding an important position amongst the Scourge offensive, with its members, most of the times, performing important tasks on behalf of the Lich King. They were often found in strongholds of other Scourge-affiliated races alongside with its local leadership, or in command of Scourge bases or operations across Northrend, acting as agents, leaders, or ambassadors.
During the Scourge's second invasion, the San'layn Prince Tenris Mirkblood had infiltrated Karazhan by orders from the Lich King, in a mission to learn about the strengths of their enemies. The operation was thwarted by experienced heroes sent by the Argent Dawn.
They took part in the assault upon the Scarlet Enclave, were responsible for the resurrection of Archmage Arugal, the creation of the massive flesh titan Thrym, challenged heroes in battle at Valhalas, and acted as ambassadors to the Vrykul of the Howling Fjord and as agents of the Scourge forces attempting to conquer Azjol-Nerub. The San'layn maintained an especially strong presence in the Borean Tundra, leading the assaults from Temple City of En'kilah and the necropolis Naxxanar floating above the fortress.
They faced heavy resistance from the Alliance and the Horde. With most of their blood princes slain by the end of the Northrend campaign, the Ashen Verdict and a group of heroes raided the Crimson Hall within Icecrown Citadel, their headquarters. The blood-princes Valanar, Keleseth, and Taldaram were raised into undeath by the Lich King to avenge themselves but were killed again, along with their blood-queen.
The Third Legion Invasion
Blood-Queen Lana'thel's daughter, Blood-Princess Thal'ena, had been imprisoned in the Violet Hold by the Kirin Tor in the aftermath of the War against the Lich King. She was freed by Lord Malgath in an attempt to strike Dalaran from within but was killed by a group of heroes.
The Blood War
With no home left, several San'layn were employed into the Horde's ranks, although Talanji did not trust them. Sylvanas came into an agreement and gave them a chance; besides having to respect the hierarchy of authority within the Horde, if the Darkfallen could work with them they would get home with the Horde, if not they would be gone.
They were tasked to clear the rubble in a collapsed tunnel that connects Nazmir to Vol'dun. Blood Prince Dreven and his followers came into conflict with Shandris Feathermoon, eventually killing members of her party. Hunted by the Alliance, the blood prince was cornered traveling via boat. Aboard The Crimson Squall he was training other San'layn and alongside his followers were siphoning power from blood-infused crystals to bolster their unholy abilities. When cornered, Dreven killed and drained the Forsaken troops aboard his ship, intending to blame their deaths on the Alliance. The blood prince and his followers were ultimately slain by Shandris and an Alliance adventurer.
The neophytes following Dreven indicated that the San'layn were growing their ranks again.
San'layn mostly retains the builds of their still-living kin. They are 5–6 feet tall with slim, athletic and strong bodies, and have fairly long hair and minimal facial hair.
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, red or dark hair. Their eyes have white, red, yellow, green, blue or black glow. Some San'layn have clawed fingers and ears more closely resembling those of a bat, and a few have developed wings.
The San'layn are cold, arrogant vampyr who inflict pain for pure entertainment. The bearer of a curse which makes them hunger for blood, they are often found siphoning from blood globes and crystals.
Darkfallen, most of the times, dress in red, orange and black garments, like tarnished versions of 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.
Their most common mount is the skeletal horse and they have been shown to keep bats, spiders, and bloodworms around them.
Skilled in a variety of magic schools, they often make use of vampiric skills, have even demonstrated some shape-shifting abilities and possess great mental influence over human minds, when empowered by the Lich King.
The San'layn oversaw the Scourge's operations throughout Azeroth, acting as agents, leaders, or ambassadors. The faction held 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, resides. The Crimson Hall was the headquarters and home of the San'layn, located inside the Icecrown Citadel, and was the place where the most powerful San'layn could be found.
Blood-Queen Lana'thel held the leading position in the San'layn hierarchy, and the Blood princes, especially the Blood Prince Council, were the second in the chain of command of the faction, often receiving direct orders from the Lich King himself. Currently, the status of leadership is unknown.
A large group of San'layn that survived the attack by the Ashen Verdict and were freed of the Lich King's control, have found a new home on the island of Mistveil that was gifted to them by the local Highborne. Currently lead by Blood Lord Sarvthus Vadvayne the San'layn of the Blighted Deadwood hunt in the night and prey on travelers that are crazy enough to stumble into the San'layn borders. The San'layn of Mistveil inhabit a city of impressive size that they have built over the time they have had since the death of the Lich King. The name of this city is San'belore, meaning 'Dark Sun' in Thalassian. The San'layn have fought for their lives against the local Bloodoak Tribe since they started making their home in the deadwood, but during the third invasion of the Burning Legion the races of Mistveil had a tenuous pact to fight the Legion and the San'layn took advantage of this. They turned on the Bloodoak Tribe by marching into the troll capital of Zul'Rashi and slaughtering their leaders while also enslaving the survivors.
- Ultimate leader of the San'layn, she was killed during the Northrend Campaign by the Ashen Verdict and Azerothian Heroes.
- Blood-Princess Thal'ena is the daughter of Blood-Queen Lana'thel. She was locked inside Violet Hold for years, suffering the gnawing pain of her eternal hunger until her cell was opened and she was slain by Azerothian Heroes.
- One of the Blood Prince Council. He is the current leader of the San'layn and has been given a chance to redeem himself as well as the San'layn by showing the Horde that he and his people can fall into line and follow the Horde leadership. If they do not, then they do not have a place in the Horde.
- One of the Blood Prince Council members who was slain by Azerothian Heroes within the subterranean city of Azjol-Nerub. His body was recovered by agents of the Scourge and he was resurrected to fight in the battle for Icecrown Citadel where he was slain once again.
- One of the Blood Prince Council members who was slain by Azerothian Heroes within the halls of Utgarde Keep. His body was recovered by agents of the Scourge and he was resurrected to fight in the battle for Icecrown Citadel where he was slain once again.
- One of the Blood Prince Council members who was slain by Azerothian Heroes and the Death Knight Thassarian atop the Dread Citadel Naxxanar. His body was recovered by agents of the Scourge and he was resurrected to fight in the battle for Icecrown Citadel where he was slain once again.
Prince Tenris Mirkblood
- Prince Tenris took up residence within the cursed tower of Karazhan as the Scourge assaulted various areas of the Eastern Kingdoms and Kalimdor. He was hunted down and killed by Azerothian Heroes.
- Prince Sandoval resides in Icecrown Citadel and challenges fighters to battle at Valhalas.
- Prince Navarius is a San'layn that can be found in Zul'Drak. He masterminded the creation of the massive flesh titan, Thrym and other flesh giants by using the corpses of local storm giants.
- Prince Atherann was one of the San'layn who once resurrected Archmage Arugal from the dead to serve the Lich King, shown in a vision for the quest "Out of Body Experience". His currents whereabouts are unknown.
- Prince Theraldis was one of the San'layn who once resurrected Archmage Arugal from the dead to serve the Lich King, shown in a vision for the quest "Out of Body Experience". His currents whereabouts are unknown.
High Priest Andorath
- High Priest Andorath is a San'layn located in the Spire of Blood of the Temple City of En'kilah in Borean Tundra.
Blood Lord Zadius
- He was a savior of many San'layn from Icecrown and lead his people to the island of Mistveil where he ordered the construction of the city of San'belore. His death came at the hands of a San'layn Assassin.
- Once a great knight of the Quel'dorei, Sarvthus was raised as a San'layn by the Lich King and is now the current ruler of the Blighted Deadwood on the island of Mistveil, as well as the Blood Lord of the cities of San'belore and Zul'Rashi.
- Once a Quel'dorei warrior, raised into undeath as a San'layn during the War against the Lich King. Until recently, he has been a nomad, slowly growing his brood. Presently, associating with the Hand of Vengeance, he is the Lord of the Sanguine Dawn.
Much of this adapted from Wowpedia