At the end of the Great Crusade we saw the Chaos Gods claiming the legions that would descend into madness. The campaign had a pretty simple system in place to track a players overall Loyalty and their victories. This system was used to determine which of the Chaos Gods would favour which Legions. In the cases of Khorne, Nurgle and Slaanesh this went pretty smoothly. For Tzeentch though we had a tie between Salamanders and Thousand Sons. In the case of a tie it would come down to a roll off between the tied Legions and any Legion that wasn't being played.
The tl:dr at the end will give a quicker overview and say where the differences with the canon occur.
On with the final events.
The Emperor's Children were forever attempting to attain perfection. This served them well during most of the Great Crusade. It served to make them an exemplar of tactical acumen lauded by their brother Primarchs. Towards the end of the Great Crusade their attempts to attain perfection took a darker note. They would forewarn their enemies of impending attack or even, on one occasion deliberately broadcast their assault plans to the enemy, all in an attempt to attain perfection. This along with the spreading rumours of the Legion's Apothecaries attempting to perfect the Astartes form, saw the Legion fall out of the Emperor's favour.
The Emperor reveals that he is stepping down from the mantle of supreme war leader of the Imperium to pursue his Great Work. In his place a new leader will take control of the Imperial war machine, the greatest of the Primarchs and the one that stood closest to his vision of the Imperium, Robute Guilliman! Where the other Primarchs had turned on each other or devolved to their own petty ideals or squabbles, Guilliman had lead a continuous series of crushing defeats against the Xenos. To honour Guilliman's achievement the Ultramarines were renamed the Scions of Guilliman.
After the shambles of the battle of One-five-four-four the Iron Hands began to make sure that such weakness would never again seep into the core of the Iron Hands. This process began slowly but in the true fashion of the Legion it escalated very quickly. It began with a review of tactical and strategic planning, then they began utilizing the forbidden tech that the techpriests had been given access to on Mars. This was then quickly followed by them unlocking their own vaults of formerly forbidden technology. As this technoarcana flooded into the Legion so to did the plan in more and more clandestine fashion. From the outside their tactics still looked as complex as a brick to the face, each was structured so that foes would be manipulated into stumbling into the path of that brick.
Masters of psychological warfare the Night Lords kept pushing the boundaries of acceptable warfare by most standards. Towards the end of the Great Crusade they had taken to focusing heavily on cutting supply lines, stealing, destroying or befouling food, water and ammo dumps and supplies. Now were populations not just scared to their wits end but they were also falling prey to famine and disease. It seemed an almost natural next step for the Legion and allowed them to choke worlds dry or back into compliance with startlingly little resources.
The bloody rampage the White Scars were leaving across the galaxy could be seen by all. Ferociously slaughtering anyone unfortunate enough to be caught in their path. Their own tactics of being a mobile hit and run guerrilla force began to shift. They were still the fastest force on the battlefield but now they would just plow straight into an enemy, and tear out the other side. Once clear they would simply turn back and repeat the process until they had claimed the heads of all their foes.
The Iron Hands are approached by their closest allies, Warmaster Guilliman and Fulgrim who attempt to use their close ties to Ferrus Manus to bring him into the fold of the Imperium Secundus. Infuriated that his two closest brothers would consider their father's empire to be falling into darkness Manus turned his back on the two, foreswearing his oaths of friendship and informing them that the next time they meet he will kill them.
-The Emperor's Children become increasingly interested in fighting the perfect war so their compliance rates significantly lower as does their favor with the Emperor.This is one of a number of similar events.
-Guilliman is named warmaster, the Ultramarines become the Scions of Guilliman
-The Iron Hands become corrupted by Tzeentch. In the canon it is the Thousand Sons
-The Night Lords become followers of Nurgle. In canon this is the Death Guard
-The White Scars turn to Khorne. In canon this is the World Eaters
-The Emperor's Children and Scions of Guilliman try to get the Iron hands to turn to their cause but the Iron Hands stay loyal to their Emperor.