The Scarlet Crusade is a quasi-religious group dedicated to the total extermination of any and all undead. As a major adversary of the Forsaken, there are many quests found in Tirisfal Glades that involve direct and not-so-direct attacks on them. The Crusade has several holdings throughout the Eastern Kingdoms, specifically the Scarlet Monastery, the towns of Hearthglen and Tyr's Hand, the cathedral found in Stratholme and various small camps spread throughout the continent.
While large numbers of the original Scarlet Crusade were annihilated by the Scourge during the destruction of the Scarlet Enclave, High General Abbendis hand-picked many Crusaders to follow her across the great sea to Northrend while the Scourge was busy destroying the towns of New Avalon and Havenshire. She then reformed the Crusaders into the Scarlet Onslaught, a fanatical army determined to take their offensive directly to the Lich King citadel and triumph at last.