This article is about both the Scarlet Crusade organization as a whole, and the 'Stormwind Outpost' represented in-game by the members of the guild <Scarlet Crusaders>.
The Scarlet Crusade is a faction of humanoids, based in the remains of Lordaeron, whose members are bent single-mindedly on wiping all undead from the face of Azeroth.
Many of the Scarlet Crusade's members are former Knights of the Silver Hand who personally witnessed loved ones struck down by the Scourge only to rise up again as mindless zombies, attacking friends and family. The flames of these experiences have been fanned by the Crusade's leadership to the point of zealotry approaching madness. Crusaders kill anyone they believe to be undead, any mortal they believe may soon be undead, any mortal they assume to be carrying the plague, mortals who stand between the Crusade and undead, and mortals who may sympathize with undead -- the latter being the way Crusaders tend to interpret any criticism of their methods. Crusaders are the sworn enemies of not only the Scourge, but also the Forsaken, despite the fact that the Forsaken hates the Scourge as much as the Scarlet Crusade does, and for many of the same reasons.
Though the Crusade's ranks are mostly human, it is not openly welcomed by the Alliance. However, its mission appeals to most within the Alliance, who generally agree that the undead need to be removed from Lordaeron so her citizens can return home. As news of the Crusaders' rising madness compete with tales of their good deeds and names of heroes amongst their ranks, especially the less informed peasants of regions far away from former Lordaeron still view this organization as a stalwart defender against the undead threat. In part, this is true, as the Crusade has indeed rescued refugees and defended civilian settlements in the Plaguelands many times, including a project to re-settle villages that have been destroyed by the Scourge during the war. Also, Scarlet Crusaders are protecting the Solliden Farmstead and its inhabitants, making it one of the few places in northern Lordaeron where the native farmfolk survived to this day.
Generally, the actions of the Scarlet Crusade are tolerated out of ignorance, and both the Alliance and the Church of Holy Light distance themselves from the Crusade's activities officially, but without trying to interfere. In fact, the Scarlet Crusade, who counts both the Alliance and the Church amongst their allies, maintains a contact to both by several emissaries - examples can be found in the Cathedral of Light in Stormwind and in the Alliance outpost of Nijel's Point.
Nevertheless, the Scarlet Crusade has killed countless refugees fleeing the horrors of the Scourge, regarding them as possible carriers of the plague or even undead sympathizers, and their growing zealotry has started to make the Crusade enemies all over Azeroth. Travelers are advised to stay clear of Scarlet Crusaders regardless of whether any undead are in the party. The Scarlet Crusade can be friend or foe, but it is generally the enemy of anyone who cannot prove that he or she is alive — qualifications and interpretation being subject to change without notice.
A lot of Crusaders seem to suffer from a dangerous form of paranoia, since they had to learn that every human might possibly be a disguised undead. After Isillien was nearly assassinated by his own page, the Inquisitor stated that if there are any doubts regarding the status of an individual, it would be better to assume it's an undead and kill it rather than risking infiltration. Although this strategy may (rightly) seem gruesome, it has successfully prevented the plague - and with it, the Scourge - from expanding all over Azeroth.
Despite its excesses, the Crusade is a powerful force in the eastern kingdoms, having already destroyed several undead strongholds in Lordaeron. They are expected to continue battling until either the Scourge are driven from Lordaeron or the Crusade itself is destroyed.
The Crusade in Stormwind Edit
In response to the recent Scourge Invasion, Stormwind had petitioned all available forces, mercenaries, groups, orders and the like, to aid in the defense of humanity's last bastion of power. The leaders of the Scarlet Crusade, sensing an opportunity for something far greater, answered that call, sending reinforcements that ultimately helped stem the tide of Scourge to the City walls and within, eventually driving them back to the Lich King's seat of power. With Stormwind owing them a debt of gratitude, a single request was made: That they allow, undisturbed, a chapter of the Scarlet Crusade to be formed within the City walls. Between the prospect of future attacks, and the debt owed to the Crusade, the beaurocrats of Stormwind conceded.
Though the exact purpose of their presence -- and their method -- is subject to some debate, the Emissaries Crowley and Zanthius have gathered a small band of soldiers both new and old to their righteous cause.
(( Information in the General section was taken from the WowWiki article, located here. ))