Magnus the Pious
If asked to make a list of the top three Emperors of the Empire of Man, the list will invariably comprise of three members: At number one is, of course, Sigmar himself. The other two slots, however, tend to be interchangeable between then-current emperor Karl Franz and Magnus von Bildhofen (Better known as Magnus the Pious), a legendary priest who assumed the rulership of the Empire during one of its darkest times.
How He Became Famous
During 2301 IC, the Old World was in some serious shit. Chaos had united under the Everchosen named Asavar Kul and began conquering everything, the Druchii decided to wage a war against their Asur brethren and Kislev was getting their shit royally kicked in. All together, this would later become known as the Great War against Chaos
When Kislev tried asking their Imperial neighbors for assistance in repelling the chaos legions, the various elector counts either ignored the requests or began amassing their own legions and fortunes in the vain hope that they would be able to weather the coming storm..if they didn't outright go full heretic. The churches of Sigmar and Ulric similarly began squabbling, as each side wanted the glory that came with leading such a vital campaign.
During this time, Magnus was a Sigmarite priest of noble birth from Nuln. His city was however under the sway of a sneaky cult of Tzeentch, but there were still those who held out hope that their manly god would save them and not some eldritch horror that might look like a blue bird. One night, the twin-tailed comet appeared in the sign, a sure omen that Magnus took as a sign to begin the uprising. Using his noble influence, he managed to assemble an army that began liberating the cities they passed through. This force grew to such prodigious size that it managed to sway the various elector counts to join forces with him. By the time he reached Middenheim, he only needed the backing of the church and asked the Ar-Ulric for his support. Said priest said "Fuck off you Sigmarite fraud!" Magnus sought to convince him to join and thus walk into Ulric's flame and left unscathed (And proved that men were better than those pansy-ass High Elves who needed mages to constantly heal them while being burned alive) he gained the total support of the church of Ulric.
Just as this is happening, two major things happened: The barbarians of Chaos ripped their way to the Empire's doorstep and Magnus met the famed elf archmage Teclis. Aware that the forces of Chaos utilized daemons not native to the mortal plane, he decided to ask Teclis for assistance in establish a magic system that would break down the overwhelming bias against any wizard while also training them to resist the influences the Dark Gods exerted. This force would come to the Kislev and help the beleagured natives and a throng of dwarfs from Karaz-a-Karak in blunting the hordes by striking the invaders in the rear. When Asavar Kul tried to escape this by splitting some of the legions to strike the gates, the Imperial forces got a sudden reinforcement unit that had tried to relieve the city of Praag that struck this detachment.
Depending on what version you go with, Magnus either met Asavar in single combat and struck him down, disarming and decapitating the warlord, or the two never met face to face and Asavar met his end when a mutinous lieutenant literally stabbed him in the back. Either way, this marked the end of the Everchosen and the effective end of the Great War Against Chaos, as the entire legion collapsed with the Kul's death and was nearly annihilated by the vengeful defenders. After Kislev, Magnus walked back to blow up Mordheim and liberate Ostland the neighboring Ostermark.
This whole event was overwhelming proof of Magnus' competence as a commander and he was unanimously crowned Emperor. His reign was considered one of the best periods to be alive in the Empire, seeing the continued purgation of the forces of Chaos, the founding of the Colleges of Magic, the establishment of Nuln as the capital of the Empire and various other influential changes. When he finally died in IC 2356, it was unusually peaceful and he was canonized as Magnus the Pious by the assembled elector counts.
Legacy
As mentioned, Magnus' reign as Emperor brought in a massive number of changes that would become commonplace during the present of Warhammer's setting.
The first and foremost of these was the Colleges of Magic, the system of sanctioned magic established thanks to Magnus' relationship with Teclis. During the times of the Great War, the Empire looked down on all forms of magic and invariably burned every one of them lest they risk the taint of Chaos. Of course, the Empire was somewhat able to tolerate the fact that the elves know magic. After the war and the understanding that daemons were very much beings of magic, it became necessary to channel that same energy to defeat them. To this end, Magnus helped the elves develop a special system of magic that enforced discipline in order to protect them from the corruption.
The second is the establishment of two major territories of the Empire: Middenland (which was formerly the divided provinces of Middenland and Middenheim, but was reunited under the Todbringers) and Talabecland (Formerly Talabheim and Talabecland, but that's more of a formality since the two territories were merged for years). This also included the establishment of Nuln as capital of the empire. Though no longer true as of the time of Franz, Nuln remains an incredibly influential and powerful city-state, being declared technically independent of the state of Wissenland despite being officially presided over by its elector-count.
Among many other small things, he also left behind a ridiculous tower in Ubersreik called Tower of Magnus. It is the tallest building in the town. A five stories tall tower housing the entirely of the town's garrison, along with a gigantic horn installed on top of the tower. The horn is said to came from a demon from the great war and it is so huge that a bellow were installed in order for people to actually blow it for emergency attack.