The Aurelian Crusades were a series of major conflicts fought in Subsector Aurelia in late M41. Subsector Aurelia, as the home system of the Blood Ravens Space Marines Chapter, was vitally important to the Imperium.
The First Aurelian Crusade
While defending planet Calderis from a sudden Ork invasion, the Blood Ravens discovered that the Orks were being stirred up by an Eldar raiding party, in the hopes of blunting the invasion of an approaching Tyranid Hive Fleet. This Hive Fleet was finally defeated on Typhon Primaris.
The Second Aurelian Crusade
The Third Aurelian Crusade
In the Space Marine video game, Captain Titus of the Ultramarines praises the Blood Raven reinforcements for their courage during the Aurelian Crusade, which is possibly the Third Crusade. There is little evidence to support this however. This line could be a simple nod toward the Dawn of War series from THQ, or it could be hinting at the timing of the game and the Crusade in question, as it has been suggested the game takes place in a time where Captain Sicarius is no longer the Captain of the Ultramarine's 2nd Company, suggesting by extension that the Third Aurelian Crusade takes place in the same time, which would mean the beginning of the 42nd Millennium (as Sicarius is still in place at the "stroke of midnight").