After Horus' successful campaign to destroy the largest Ork WAAAGH! known in the history of the Imperium to that point at the world of Ullanorduring what became known to later Imperial historians as the semi-legendary Ullanor Crusade, the Emperor declared it the greatest victory of the Imperium to date. Horus' reward was the title and position of Imperial Warmaster and with it the supreme command of all of the Imperium's armed forces, Space Marines and Imperial Army starships and troops alike.
After doing so, the Emperor made it known that He was needed back on Terra, where he would undertake a secret project intended to open the next phase in human cultural evolution. What the Emperor intended was to make use of the ancient artefact known as the Golden Throne, an instrument of technology created before the Age of Strife that had been unearthed on Terra that acted as a powerful psionic augmentation device, to open the Aeldari Webway to the use of humanity. This would allow all the worlds of the Imperium to be instantaneously interconnected through the Webway in a way that would make travel by starship obsolescent, and also end humanity's need to rely on psykers for interstellar navigation and communication with all the risks this entailed. The creation of a human Webway was intended to be the Emperor's greatest gift to Mankind and usher in millennia of progress and peace once the Great Crusade was complete.
Horus was not informed of the Emperor's plans on Terra, and felt deeply troubled by his father's sudden unwillingness to confide in him, once his closest confidante, and by his decision to retreat to Terra with the Great Crusade yet unfinished. To make matters worse, while some of his fellow Primarchs accepted his promotion, others, chiefly Angron, Konrad Curze and Perturabo, openly begrudged Horus his new authority.
There was also the widespread erroneous belief among the Astartes that the Emperor would disband or reduce the Space Marine Legions to the level of peacekeeping forces once the Great Crusade was completed. Horus resented his brothers' feelings and thought that while he was winning new worlds for the Imperium the Emperor was handing the Imperium he, his brothers and the Astartes had won to corrupt mortal bureaucrats and Terran nobles who knew nothing of honour or sacrifice. These existing seeds of bitterness, jealousy and pain were all the Ruinous Powers of Chaos needed to sow the seeds of dissension in the Warmaster's mind.
Events came to a head following Horus' wounding at the end of the Great Crusade during a battle on the moon of Davin against Nurgleite undead by the Chaos-tainted weapon known as the Kinebrach Anathame that had been wielded by the Nurgle-corrupted former Planetary Governor of Davin, Eugen Temba. Even Horus' superhuman immune system could not defeat the terrible Nurgle-created toxin that had laced the blade of the Anathame, and in their desperation to save his life, the Captains of the Luna Wolves gave in to the machinations of the Word Bearers Legion's First Chaplain Erebus and gave Horus into the keeping of the Chaos Sorcerers of the Temple of the Serpent Lodge on Davin.
As part of their "healing" ritual, the sorcerers sent Horus' mind into the Warp, where Erebus, the First Chaplain of the Word Bearers Legion who had long served Chaos, acting as the agent of the Dark Gods, sought to corrupt him. Erebus used a deceptive vision of the future of the Imperium where the Emperor was worshiped as a god to convince Horus, who was filled with jealousy at the sight of thousands of pilgrims worshiping in a grand cathedral dedicated to the Emperor, to replace his father as the master of the galaxy.
When Horus awakened fully healed, he awoke already corrupted, ready in his terrible ambition to turn on his father and attempt to wrest control of the Imperium from the Emperor no matter the cost, unleashing the terrible interstellar civil war known as the Horus Heresy. In return for his allegiance to their cause, the Chaos Gods offered Horus the power of the Warp, cementing the corruption of his soul.
Horus would never realise that the dark future of the Imperium that the Chaos Gods had shown him, where Mankind was oppressed by superstition, ignorance and constant war, would be caused by his actions and his betrayal of his father. The Heresy would end with the death of Horus, the exile of his 9 Traitor Legions into the Eye of Terror, and the quasi-death of the Emperor of Man following His internment within the Golden Throne. This was ironically the very instrument through which He had hoped to open the Webwayand create vast new vistas of progress for Mankind.