The Argent Dawn History
After the death of the Scarlet Highlord Alexandros Mograine, the corruption of the Scarlet Crusade became apparent to some of its members through High Inquisitor Fairbanks, who spoke of the corruption (for which he was later murdered). Those who paid him heed left the ranks of the Scarlet Crusade and established the Argent Dawn, to protect Azeroth from the threat of the Scourge without subscribing to the blind zealotry of the older faction.
While they share the same goals as the Crusade, the Argent Dawn opened its ranks to not only other Alliance races besides Humans, but also members of the Horde and even some of the Forsaken (but only if they proved themselves to be good, and were no longer members of the previous organization). They caution discretion and introspection, and put a lot of emphasis on researching the Scourge and how to combat them.
With time, the Argent Dawn has grown diversified. A subfaction of the Argent Dawn, the Brotherhood of the Light, works to keep relations between Argent Dawn and Scarlet Crusade from growing violent.
Despite their influence, the Argent Dawn do not have the manpower of the Scarlet Crusade, and might not be able to stand alone against the Scourge. Currently, the Argent Dawn and the Scarlet Crusade have formed an uneasy alliance in order to focus their combined strength against Kel'Thuzad's forces in the Plaguelands.
However, corruption looms within the ranks of the Brotherhood: when a player has looted The Phylactery of Kel'Thuzad from Naxxramas, he is able to turn it in to Father Inigo Montoy, a member of the Brotherhood, in Light's Hope Chapel, who then promises to deliver it to the Argent Dawn in order to ensure the final death of the Lich Lord. As it later turns out, the phylactery has never reached its destination — so either Father Montoy or a powerful individual within the Argent Dawn is following a darker scheme ... or even secretly working for the Scourge.