After Lord Uther the Lightbringer was betrayed and killed by his former apprentice Arthas Menethil, the Knights of the Silver Hand was decimated by the Scourge's undead armies.
Isillien was formerly a priest of the Holy Light in Lordaeron, assigned to be the liaison for the Knights of the Silver Hand. There he worked closely with the elder High General Abbendis (the father of the younger Abbendis) and his youthful charges, tutoring the young paladins in their quest for truth. Priests and paladins fought as the Scourge attacked the city, but they fled as Lordaeron fell around them. Some whisper it was the wounds both men received in battle, or perhaps just the shock of seeing their whole purpose destroyed, but since that day both Isillien and Abbendis had become quite insane. Isillien and Abbendis wandered the area as the Scourge defiled the land, destroying what undead they could, gathering what warriors would join them. They had but one concern: eradication of all that had destroyed Lordaeron.
Among the group were the high-ranking officers Isillien, the Abbendises (father and daughter), Alexandros Mograine & his son Taelan Fordring and Saidan Dathrohan have been said to have helped found the Scarlet Crusade, an order of knights focused on the singular goal of defeating the Scourge in all its forms.
Taelan Fordring, one of Isillien’s protégés, adored his tutor and his general. He offered his family’s keep and land, untouched by the undead, for their base of operations. The Crusade gained members whose lives had been ruined by the Scourge. Most members have seen a loved one die and, more often than not, be raised to fight alongside her murderer. Isillien became the spokesman for the group, his ravings gaining more followers than repelling them — and who could argue with the man that the undead were evil?
Some had reason to pause when Isillien’s young page was found with Isillien’s knife through his heart, and the priest calmly reported that the page had died that afternoon while hunting, and the undead had raised him and sent him back as an assassin. He was so convincing that the incident cemented the fear for most: if a freshly killed and raised undead can look just like a regular human, they had no idea who among them may or may not be with the Scourge. After a month of quarantine, all priests and warriors with the Crusade were declared clean. The elder Abbendis and Isillien felt that although they had a good base, they really needed to attack the Scourge where they were the thickest: in the Eastern Plaguelands. Isillien took on the title of Grand Inquisitor and control of the Western Crusade, while the elder Abbendis led the majority of the forces to Tyr’s Hand in the east to strike at Stratholme. As they established their bases and took in many more warriors, they destroyed any undead they found.
Frequently, groups of refugees fleeing the undead also fell beneath their furious swords. As the Grand Inquisitor claims, one cannot be too careful. All undead — good, evil or neutral, Scourge or forsaken — are the rivals of the Scarlet Crusade. The undead are abominations and must be destroyed before the land and the people can heal. They count the Alliance and the Church of Holy Light as their allies, but these organizations quickly distance themselves from these fanatics, and instruct all travelers to give a wide berth to any Crusaders.
Not long after its formation, the Crusade attempted a strike against the Scourge base in Northrend, and 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.