In the fictional Warcraft Universe, The Scarlet Crusade is an organization of religious zealots who combat the undead (Scourge and Forsaken alike) that control Lordaeron. Their mission is the complete eradication of the undead from Lordaeron, so that it may be reclaimed by the humans and restored to its past glory. The Crusade was formed in the aftermath of Lordaeron's fall by Highlord Mograine, Grand Inquisitor Isillien and High General Abbendis. They are currently led by Grand Crusader Saiden Dathrohan. In the course of eradicating the undead, the Crusade has killed countless innocents suspected of carrying the plague of undeath. In their madness, the Crusaders have failed to recognize that they are following the same path that led to Arthas's fall from grace.
History of the Crusade This information is taken from WoWWiki:
After Sir Uther Lightbringer was betrayed and killed by his former apprentice Arthas, the Order of the Knights of the Silver Hand was decimated by the Scourge's undead armies and ceased to exist.
The few surviving Knights of the Silver Hand remained in Lordaeron, destroying what undead they could. Among the survivors were the high-ranking officers Isillien, Abbendis and Mograine. They recruited warriors to their holy cause of eradicating the undead. They were soon joined by the paladin Taelan Fordring, one of Isillien's protégés, who offered his family's keep and lands as a base of operations for the Crusade.
After a month-long quarantine of the warriors and priests to ensure that there were no Scourge infiltrators, the Scarlet Crusade began hunting the undead in earnest. Abbendis took most of the Crusaders with her to Tyr's Hand in the Eastern Plaguelands, from which the Crusade struck at the undead controlling the city of Stratholme, once the headquarters of the Knights of the Silver Hand.
Not long after its formation, the Crusade attempted a strike against the Scourge base in Northrend. They built a fleet specifically for that purpose. The attempt failed, although the Crusaders reached as far as the mouth of Icecrown Glacier. There were many casualties among the high ranks of the Crusade, including its first and only Grand Admiral, first Captain-General and first Chief Assassin.
There are those that claim that the Scarlet Crusade now has a more sinister purpose, that it is being used for darker reasons than the honorable - if misguided - cause of clearing the Plaguelands from the taint of the Scourge. This may be correct, as not only was Highlord Mograine betrayed and murdered by his own son, but also the Crusade is actually headed by the Dreadlord Balnazzar, disguised as (or possessing) the Grand Crusader, Saiden Dathrohan.
Some Crusaders who have begun sensing the corruption within the Crusade have left the ranks of the order to found the Argent Dawn. These warriors are generally regarded as traitors by the Scarlet Crusade, especially since the Argent Dawn also counts Forsaken amongst its ranks - but Kel'Thuzad's recent assault on Azeroth has forced the two organizations into a reluctant alliance with each other. Both sides have been able to shelve their mutual dislike in order to fight back the new horrors of the Scourge in Naxxramas, but tensions are high and there is only a superficial allegiance between them.
Most recently, the Scarlet Crusade has forged a temporary truce with the Argent Dawn and are being kept in check by Tirion Fordring and his knights. This uneasy pact was made in order to repel the increased Scourge activity and to depose Kel'Thuzad from his throne in the necropolis of Naxxramas, a task that neither the Argent Dawn nor Scarlet Crusade could accomplish on their own. Adventurers can meet on friendly terms with emissaries of the Scarlet Crusade in Light's Hope Chapel where they will share their secrets with those who perform certain tasks for them.
Organization The ranks of the Scarlet Crusade have been estimated at more than ten thousand. Membership is comprised chiefly of humans of the warrior, paladin, priest, mage, and hunter classes. The Crusade is organized along military lines: High General Abbendis commands the military operations, while Grand Inquisitor Isillien leads the priests in the questioning and torture of suspected undead.
Most Crusaders are based in Tyr's Hand where Isillien has declared the Eastern Plaguelands to be the focus of the Crusade's attacks. In the ruins of Stratholme, the Scarlet Crusade has established a more or less permanent foothold from which it wages constant battle upon the Scourge within the city. They also maintain a stronghold at Hearthglen, and in eastern Tirisfal Glades the Scarlet Monastery serves as a training and research center.
Crusaders that have battled undead rise quickly through the ranks. Dissent is not tolerated, for the Crusaders cannot understand how any reasonable human might be opposed to the Crusade's actions against the undead. As a result, while the Crusaders may not be in complete agreement over how they accomplish their goals, they are kept in line by fear of their superiors' wrath.
Bases of operation
The Scarlet Crusade operates several bases throughout the Plaguelands and Tirisfal Glades.
Hearthglen Located in the north of the Western Plaguelands, this rural community was once the Fordring family's ancestral holdings. It was offered by Taelan Fordring to the Crusade for use as the headquarters of the Western wing of the Crusade. It is currently commanded by Grand Inquisitor Isillien and Highlord Taelan Fordring.
The Scarlet Monastery Located in Tirisfal Glade, the Monastery served as a center of knowledge, learning, and the Holy Light in Lordaeron before the Third War and the coming of the Scourge. With the rise of the undead, it was converted into one of the Scarlet Crusade's major strongholds. It is currently commanded by High Inquisitor Whitemane and her champion, Scarlet Commander Mograine.
The Scarlet Bastion
A small contingent of Abbendis's troops, led by Grand Crusader Dathrohan, managed to wrest control of Stratholme's cathedral from the undead roaming the ruined city. From there, they strike against the undead controlling the rest of Stratholme. Unbeknownst to the Crusaders, Dathrohan is hiding a terrible secret...
Tyr's Hand A small community in the Eastern Plaguelands, it serves as the main base of operations for the Eastern wing of the Crusade. High General Abbendis took the majority of the Crusade's soldiers there in order to strike at the Scourge-controlled city of Stratholme.
Enemies The blind zealotry of the Scarlet Crusade has made them far more enemies than the Scourge alone. They distrust all non-human races, pitting them against most of Azeroth. They are especially aggressive towards the Forsaken, who they believe to be no better than the mindless Scourge, and the rest of the Horde that supports them. Even the Alliance has turned their backs on the Crusade. All players are at war with the Crusade and the only known way to reach a friendly status with them is to recover a weapon called Corrupted Ashbringer from Naxxramas Lair.
Alleged Necrophobia It is widely believed by most Azerothians that members of the Scarlet Crusade are victims of necrophobia. This is largely due to the fact that Crusaders have a strong hate for the dead, despite being at war with almost every other faction on Azeroth.
Notable Members of the Scarlet Crusade
Leadership, Upper Echelon * Grand Crusader Dathrohan * High General Abbendis * Grand Inquisitor Isillien
Others * Archivist Galford * Arcanist Doan * Rohan the Assassin * Interrogator Vishas * Foreman Jerris * Foreman Marcrid
Trivia * Though the Crusade is said to hate all non-human races, the Armory's Hall of Champions displays statues of deceased or missing high-ranking members. Among them are two High Elves and a Dwarf.