Horus - Signus Prime: When the Imperial Warmaster Horus, the greatest of the Emperor's genetic sons, fell to the temptations of the Chaos Gods and became swayed by their promises of power, he sought to sway his brother Primarchs to his cause. Horus and Sanguinius, the Primarch of the Blood Angels Legion, had fought many campaigns by each other's side. Their relationship was so close it had even incited jealousy amongst their brother Primarchs on occasion. But in his black heart Horus knew that Sanguinius would never willingly betray their father, and so he had formulated an audacious plan to either convert the Blood Angels Legion to his cause or utterly destroy them. To this end, Horus had discovered decades earlier a carefully guarded secret of the Blood Angels when he fought alongside Sanguinius' IX Legion in a xenocidal campaign on the world of Melchior. Horus had come upon his brother Primarch in a sunken ruin of an alien chapel and had witnessed the unthinkable -- Sanguinius murdering one of his own Astartes. Sanguinius explained his actions to his bewildered brother. He had discovered that within his own genome there was a trait that lay buried and waiting to be awakened. This genetic flaw would later be known as the Red Thirst.Sanguinius had been aware of the flaw in his genome for several years, keeping the truth from the Emperor and his fellow Primarchs. Some of the Angel's sons had learned a measure of the truth, but only Azkaellon, First Captain Raldoron, the IXth Legion’s Master Apothecary on the Legion homeworld of Baal  and a few others were fully aware of the extent of this affliction. They were united with Sanguinius in finding a way to repair this Flaw. Horus swore to his brother that he would never speak of this matter to anyone, even their father. He would keep this promise for as long as Sanguinius wished him to. The Angel was touched by his brother's gesture and expressed his gratitude. Horus solemnly vowed to help Sanguinius deal with this matter, however long it took. Little did they know at the time, that one day a corrupted Horus would take full advantage of this knowledge and attempt to turn the Blood Angels' Flaw against them.Decades later, Horus exploited this knowledge of the Blood Angels' genetic Flaw. The Warmaster had found a way to sway his "beloved" brother's Legion to his cause and the service of the Chaos Gods. In his capacity as Imperial Warmaster, Horus ordered Sanguinus to gather his entire Legion and make for the Signus Cluster, a triple star system located in the Ultima Segmentum near the Eastern Fringe. His IXth Legion was to cleanse the 7 worlds and 15 moons that comprised the Signus Cluster of xenos invaders and release the humans settled there from their xenos-overlords. To further entice Sanguinius, the Warmaster informed him that he had found the means by which the Blood Angels would be able to excise the darkness from within their souls, and rid themselves of the Flaw. If Sanguinius obeyed the Warmaster's command in this matter, then Horus promised him that the Blood Angels would find a new freedom. Sanguinius had no reason to doubt Horus, for they were as close as two brothers could be. Many of their fellow Primarchs were even jealous of the closeness between the pair. Sanguinius relished the opportunity to once again prove the value of their bond, and so the Blood Angels Legion gathered in its entirety and duly set course for the Signus Cluster, unaware that they were heading into a terrible trap.Unaware of the Warmaster's perfidy, Sanguinius willingly obeyed his brother Primarch and immediately set out for this volatile region of space. Unbeknownst to the Blood Angels, they were blindly walking into a deadly trap, for the Signus Cluster had fallen prey to agents of the Ruinous Powers and become a veritable Realm of Chaos -- a system of hellish Daemon Worlds under the rule of a Greater Daemon of Slaanesh known as Kyriss the Perverse. When the Blood Angels arrived in-system, their fleet was ambushed by the malevolent forces of the Warp, crippling or killing many of their Navigators and Astropaths  in the initial onslaught. The Blood Angels now faced the fury of Chaos for the first time. Kyriss sent an image of himself to Sanguinius, declaring his lordship over the system in the name of Slaanesh and taunting the Primarch into taking it back from him. Though they had never faced such a foe, the Blood Angels prepared their counterattack, sure that they would prevail. Rising to the challenge of the vile Greater Daemon, the Blood Angels Legion attacked the daemon host of Kyriss, launching a series of attacks across the seat of the daemon's power, the world of Signus Prime.During the epic battle, Sanguinius came face-to-face with a new nightmare known as Ka'bandha, a Greater Daemon of Khorne. During the battle that ensued, Sanguinius was sorely wounded and temporarily incapacitated. He witnessed Ka'bandha slaughter 500 of his sons with huge swathes of his mighty axe. The psychic backlash of the deaths of so many of his sons blasted Sanguinius into unconsciousness. With the fall of their Primarch and the slaughtering of their brethren, the Blood Angels Legion was consumed by a black rage that drove them into a berserker's fury as they charged into the daemonic horde and in their madness they smashed the horde of daemons asunder. Yet the brutal violence of the daemon Ka'Bahnda had unleashed something dark within the psyche of the Space Marines, a thirst for blood that would not be slaked until every taint of Chaos had been erased from the planet. Even the mighty Kyriss was banished back to the Immaterium. Only when the planet was cleansed did the rage of the Blood Angels finally subside. Though Signus had been freed from its thrall to the Forces of Chaos, the cost of victory was far higher than any could have wished. The berserker rage the Blood Angels had experienced had left a brooding shadow on their souls that would manifest in the centuries to come as the great curse (later known as the Black Rage) that would afflict the Blood Angels and their later Successors.