|This article contains spoilers! You have been warned.|
Born on Eidolica, by the time of WAAAGH! The Beast, Thane was Captain of the Fists Exemplar's Second Company. When the Orks came to Eidolica, Chapter Master Alameda and the First Company were wiped out in the invasion, and Thane was selected to become Chapter Master. While preparing to lead the Fists Exemplar in a last stand of their now survivor less homeworld, they got a message from the Black Templars informing them that "the Last Wall" had been activated. Rather than stay and die on their homeworld, Thane decided it would be better to go and die defending Terra. Heading to Phall with other Imperial Fists successors, Thane and company learned that Slaughter Koorland was not merely the Chapter Master of the Imperial Fists, but the only surviving member. Though horrified, Thane and the other Chapter Masters subordinated themselves to Koorland, reforming the VII Legion to defend Terra, with Thane and High Marshal Bohemond of the Black Templars becoming Koorland's chief advisors. Arriving to Terra in force, the Last Wall was eventually able to overwhelm Ork forces on the orbiting attack moon and shut it down. Expecting to be hailed as heroes, the Marines were surprised to find that Lord Commander of the Imperium Udin Macht Udo denounced them and took all the credit for destroying the Attack Moon. Despite wanting Thane to show Udo FIRST HAND what a Space Marine was fully capable of, Koorland had him stand down, then sent the Fists Exemplar on a mission to figure out why the Soul Drinkers were dicking around and hadn't responded to the Last Wall message.
After Koorland was convinced to supplant Udo as the new Lord Commander, Thane lead the Fists Exemplar to Mars to retrieve Magos Eldon Urquidex. You see, the Adeptus Mechanicus had been up to some heresy in trying to commandeer Ork teleportation technology so they could use it to escape the Sol system and get out of the Imperium, and Urquidex was a witness to this. The Mechanicus was in the process of turing Urquidex into a Servitor to shut him up. After a brief confrontation which was totally not a micro-civil war when the Skitarii definitely did not shoot down a drop pod, Thane took Urquidex into custody. Thane then participated in several other missions that made the myths of the Great Crusade and the Horus Heresy real: learning that Vulkan lives, recovering the last Sisters of Silence, and the first and second battles of Ullanor Prime. During the first battle, Vulkan apparently sacrificed his life to stop the Beast, but it didn't work. And during the second battle, which tried to use a captured Weirdboy to create a "reverse-WAAAGH!" effect to kill the Beast, Thane bore witness Koorland being killed by the Beast, and the extinction of the Imperial Fists.
Trying to rouse himself out of depression, Thane decided to hold an impromptu Feast of Blades. As the Fists Exemplar champion who had won the last Feast had died on Eidolica, Thane took his place as champion, and subsequently won the Feast of Blades.. It was during the pain of the Feast that he had a revelation, like Rogal Dorn did with his magic pain glove: the Imperial Fists could live on in the successor chapters. Naming himself as the new Chapter Master of the Imperial Fists, Thane folded the last 73 members of the Fists Exemplar under his command into the new Imperial Fists, with the other successors donating some as well. Allying with Assassinorum Grand Master Drakan Vangorich, he decided to take a different approach to the High Lords of Terra: fuck them. Forcing the Lord Commander Militant of the Imperial Guard and the Lord High Admiral of the Imperial Navy into going along with a plan to attack Ullanor a third time, Thane deployed most of the new Imperial Fists into a hollowed out asteroid and sent it and several others flying toward Ullanor as a method of Exterminatus (You mad, Adepts?). Escorting the Sisters of Silence to the Beast's palace at Gorkogrod, Thane bore witness to the last Sister commit suicide to cause a Weirdboy's head to explode, killing every Ork on Ullanor.
After reluctantly agreeing to become Lord Commander of the Imperium, Thane announced several decrees: One, no more closed session meetings of the High Lords. Two, the Fourth Founding of Space Marines would occur. And three, the Imperial Fists were to become a Crusading Chapter, leaving Terra to go find the enemies of man before they could come and screw up Terra. With his departure, he named Vangorich as Lord Protector of Terra. It was after he left that he received some disturbing news: the Fists Exemplar under First Captain Zerbyrn had allied with the Iron Warriors while fighting the Orks after losing contact with Imperium. This was bad enough, but Thane learned that Fists had killed Imperial servants in defense of their temporary allies, including Bohemond of the Black Templars, before finally submitting themselves to the Iron Warriors with only one company of Astartes being able to return as loyalists and joining the Imperial Fists. Enraged, Thane ordered all evidence of the Fists Exemplar removed, consigning his former Chapter to oblivion and extinction. By comparison, he took the Beheading much better, deciding that High Lords had fucked up too much to be trusted and thinking Vangorich would be useful in rebuilding the Imperium. And for eighty years, he was right: the Fourth Founding occurred, the Orks were driven back, and new Guard regiments were raised.
Then, problems began emerging. Populations were relocated for no reason. Incompetent planetary governors were appointed. Worlds starved as resources were redirected to Terra for vain projects. It became all to clear that in the last twenty years, Drakan Vangorich had lost his mind. Thane joined a strike force to remove Vangorich, but subordinated himself to the Chapter Master of the Sable Swords, feeling his judgment regarding Vangorich was compromised. It became moot when Vindicare assassins took out the officers, leaving Thane commander by default. Following Vangorich to the Eversor Temple, Thane and the combined force of Space Marines faced down 100 Eversor assassins. In the end, only three people were left: Thane, Vangorich, and an Eversor. Too wounded to effectively fight any longer, Thane was saved only when the Eversor revealed itself to be Esad Wire and turned on Vangorich, allowing Thane to Blam the mad Grand Master.
In short, if Slaughter Koorland was the hero the Imperium needed, then Maximus Thane was the hero it deserved and Drakan Vangorich was the 'hero' that it got.
One small oversight comes from the Siege of Terra book "The Lost and The Damned". With Maximus Thane serving as Captain of the 22nd Company, meaning by the War of The Beast, he would, at minimum, be 1500 years old, so basically Dante's age. Which means he doesn't even care about old age, cause by the end he has a hundred year long crusade. Of course, it's also possible that Heresy-era Thane and Beast-era Thane are two different guys, and the latter is simply honoring the memory of the former by using his name.