The Crow Legion were marines from The Unseen Ravens chapter, descendants of the Raven guard and declared excommunicate in M36 following extended exposure to the warp. They are now marines of Tzeentch with a high concentration of Psykers. Very skilled in divining the future and staying unseen, the Crow Legion is a forced to be reckoned with.
The original origins of this warband span back to M36, during the Cursed founding. Trying to create stealth troops with an unparalleled ability to stay unseen. To do this they used the genetics of the Raven guard Legion, tempering with genetics to harness the ability of Corvus Corax, the power to turn invisible. The experiment was somewhat successful, using advanced genetics found in some creatures to make the eye difficult to focus on the subjects (creating what the Adeptus Mechanicus called a "someone else's problem" field). Due to this it was deemed a success and the geneseed went in to full production.
This new chapter, known as the Unseen Raven's chapter were given the home world of C'rowe and a battle barge nicknamed "Raven's call". The chapter served with distinction on a variety of worlds, their stealth tactics and curious ability serving them true on a myriad of worlds. They were lauded as inspirational by many, however they were destined to fall...
During the war in the Angelos system during M39, the Unseen Ravens faced their toughest challenge against the Thousand sons and their Tzeentchian cultist allies. However they fought stoically quashing the enemies of the imperium and destroying the Cults and chaos marines wherever they went. In the strike forces final stand, they planned to summon a Powerful Daemon of Tzeentch, one known as Kairos Fateweaver. Storming the ritual, Chapter master Kharvon and elements from the 1st, 3rd and 5th companies, along with their captains: Kharvae, Shrake and Zealo respectively. Also accompanied by Chief Librarian D'kestril, a secret follower of Tzeentch who manipulated the chapters fate for over 600 years.
The battle was a massacre, the warp portal at the centre of the battle, causing numberless horrors, screamers and flamers spilling out into reality. Finally getting to the center, Kharvon finally slaughtered the coven of sorcerers that kept the portal open. However, instead of it slamming shut like they were told by D'kestril, it grew wider, engulfing the strike force and trapping it in the eye of terror. In reality it shouldn't of happened, however having waited for 600 years, D'kestril manipulated the warp portal ever so slightly.
Descent into Darkness
Slowly but surely, the marines began to mutate. One by one. The mutations were varied in appearance, except they all shared 2 similarities with each other, one, each marine's fingers began to mutate into black talons, and slowly but surely each marines back grew a thick plume of black feathers, for some even spreading to their arms and legs. Worse still, whereever they went they were followed by scores of black ravens, attacking them, and whereever the ravens went, the daemons of Tzeentch weren't far behind.
D'kestril pronounced this as the Emperor forsaking their once proud chapter, twisting their minds into bitterness and anger. Chapter Master Kharvon, however, knew the truth and made it his duty to try to venture out of the warp and into realspace, trying to rejoin the Imperium that they had been torn away from. He succeeded eventually, bring 40 marines (17 tactical marines, 4 terminators, 7 scouts, 5 assault marines, 6 veterans of both sternguard and vanguard and one of the captains: Zealo of the fifth) from the eye of terror, where they were summarily destroyed from orbit as they neared Cadia. Hearing the screams of his comrades as they were dragged back to the warp was the remaining marines, who took it as firm proof that the Imperium of Man had forsaken them completely. At the prompting of their new leader, the Chief Librarian D'kestril, they repainted their armour and dedicated themselves to Tzeentch. Once doing so the daemons stopped and dissapeared and the crows rested around them, cawing with one voice from a thousand throats. From that moment on the Unseen ravens were no more, now they were the the Crow Legion.
During a particuarlly viscous battle early on in the career of the warband to take the planet of Corviahe, the Crow Legion suffered crippling losses, losing a large amount of man power and ending up with only 80 marines remaining. Presumably taking exception to their crude ways, Tzeeentch cursed the warband to hear the spirits of every member that has died, forcing them to feel the guilt for every member and never forget. Due to this, the chapter used many of the cultists as stock for new marines, tripling their numbers from the original 200 to around 600. Since then the entire warband has taken to using more subtle methods, using the ways of assassination, sabotage and terror.