In my campaign, the Magus PC has a Dark Secret and masquerades as his "twin brother" who is a Knight and the Bastard son of the Viscount of Avranches/Earl of Chester. He joined the 1st Crusade under Robert of Normandy. The player regularly interfered with various sieges of cities and various battles, making the 1st Crusade wildly more successful, culminating in the additional capture of Aleppo and the towns all the way down to Damascus. The player is now the "Prince of Damascus", and making war with the Fat Peasant of Bosra.
In addition, the PC pushed the Hermetic Wizards of the Crusade into declaring open warfare on the Order of Solomon.