The Crimson Path is the overall strategic plan of Abaddon the Despoiler, the Warmaster of Chaos and Chaos Lord of the Black Legion, to lead the Forces of Chaos from the Eye of Terror all the way to Holy Terra itself in the final days of the 41st Millennium. Abaddon intends to succeed where his predecessor Horus failed, breaking the Imperial Palace wide open and tearing the Emperor's rotting corpse from the Golden Throne. Over the millennia, each of his conquests has gradually paved the way from the Eye of Terror towards the Sol System. Less obvious, however, is that along that path, his actions have gradually weakened the veil between realspace and the Warp, leading to the birth of the Great Rift after successful conquest of the Fortress World of Cadia during the 13th Black Crusade in 999.M41.
During the course of his long years on the threshold of the Eye of Terror, Abaddon discovered a strange truth about the barrier between the Materium and Immaterium. Dotted across the galaxy were ancient structures of black stone, stark and monolithic on the outside but extremely complex within. There were many of these upon the Fortress World of Cadia. Known as "pylons" to the planet's militarised populace, they were so old that none gave them more than a passing thought. It was Abaddon's belief that these structures were the very things that maintained the stability of the Cadian Gate -- something about them kept the energies of the Warp from spilling through the Eye of Terror into realspace.
For thousands of Terran years, the Despoiler sent his agents across the galaxy to locate these monoliths of black stone. He spent many years calling in old alliances, striking daemonic bargains and invoking ancient pacts with the Traitor Legions and their corrupted Daemon Primarchs. Thread by tenuous thread, the Despoiler put together a plan to isolate and destroy these structures, using false objectives or even entire Black Crusades to conceal his true motives. Over the course of several world-scouring invasions, Abaddon shattered, toppled or blasted apart these structures wherever he found them -- sometimes in person, at other times with his agents, his flagship the Planet Killer, or the immense Blackstone Fortresses he co-opted during the Gothic War.
The last of and greatest of these destructive campaigns was levelled at Cadia itself. Abaddon's 13th Black Crusade brought about so much death and destruction that his daemonic allies could breach realspace wherever they wished and be sustained indefinitely by the unbridled mayhem that raged around them. The demolition of the strange black monoliths scattered across Cadia's surface was finally achieved, the capstone of a grand design that had seen the cataclysmic events of Pandorax, the Siege of the Fenris System, the return of the Planet of the Sorcerers to realspace and operations on a dozen worlds besides tied together into one galaxy-wrecking whole. Suddenly, all the manifold armies of Chaos were free to pour into realspace like blood from a deep wound. True to his claims, Abaddon had succeeded in ripping open the gates of hell.
As the galaxy has split along its length, with Warp Storms raging in a hundred locations and more, those Traitor Legions and Renegade Chapters who had sworn allegiance to the Warmaster of Chaos formed the vanguard of the invasion. They are a broad-headed spear that has plunged deep into the heart of the Cadian System and emerged from the other side. Abaddon cares little how many casualties are amassed as his great agenda comes to fruition. His only concern is that enough blood be spilled to saturate the battlefields of the Imperium's defenders, and in doing so summon more daemonic armies to his cause. This is a strategy Abaddon intends to see replicated on world after world as he extends the Crimson Path, each planetary invasion taking the Chaos Space Marines closer to Terra, with the armies of the daemon lords following in their wake.
With Cadia reduced to ruin, the Great Rift which has opened across the galaxy, the Cicatrix Maledictum, is being effectively weaponised by Abaddon's invasion. It spills out behind the 13th Black Crusade, roiling and growing more violent in Abaddon's wake, agitated by the scale of the Black Legion's many slaughters. Simultaneously, dozens of Renegade Chapters from the Warp rift once known as the Maelstrom are pouring out into the Segmentum Solar, forcing the Space Marines and their Primaris reinforcements to spread themselves thin as they fight on hundreds of fronts. Abaddon has driven a talon of unreality into the Segmentum Solar, its onset heralding the doom of the Imperium. The High Lords of Terra live in constant fear of realspace collapsing entirely, and are sending every military force they can muster to halt Abaddon's rampage. As the drama plays out to its desperate conclusion, one thing remains certain -- unending war.
- Codex Heretic Astartes - Chaos Space Marines (8th Edition), pg. 16
- Codex: Chaos Space Marines (6th Edition), pg. 20