From wh40k wiki. The raven guard does have a genetic condition which matches with the behavior of the carcharodons. Also, look at the pre corax Raven Guard, when they were led by Shade Lord Arkhas Fal. They were brutal slavers then. Which the sharkies also happen to be.
Raven Guard | Warhammer 40k | FANDOM powered by Wikia
Shadow Killers - Amongst the Raven Guard and the XIXth Legion that preceded them, there was a tendency amongst combat veterans placed in situations of extreme stress to forsake all pretense of self-preservation, fighting with a terrifying, silent determination and the sole aim of causing as many casualties and as much destruction as possible before succumbing to their foe. This strange blood-soaked ennui that drove many amongst the Raven Guard to sacrifice themselves in battle was known among the Terran XIXth Legion as becoming "Ash Blind", while the Raven Guard native to Deliverance knew it as the "Sable Brand", a condition heralded by the darkening of the entire eye to black. Those warriors who survived their suicidal furies sometimes recovered, their brethren never mentioning the incident in a typically Raven Guard display of reticence, but more often were evermore plagued in combat by the urge to slay until death took them, paying no heed to tactical plans. The XIXth Legion, in the days before Corax' return, made little attempt to organise the Ash Blind on the battlefield, relying on the flexible nature of their tactical doctrines to make the maximum use of their suicidal urges when they took hold. The Raven Lord turned this aberrant effect of his gene-seed into a valuable weapon in his tactical arsenal, forming those afflicted by the Sable Brand into units of Shadow Killers and employing them as shock assault troops and assassins. It was the employment of such troops that allowed Corax to best his brother Roboute Guilliman in several simulated battles, an unexpected tactic that the lord of the Ultramarines was quick to adopt into the Legiones Astartes order of battle, creating the Moritat as it was later known amongst the Legiones Astartes.
Corax banished the terran legionaries under Arkhas Fal into the void when he took control of the legion. And made no effort to ever recall them, as their modus operandi was completely at odds with his own beliefs. From what I can see on wiki, their chapter master rank is Shade Lord.
Last edited by Brother Lucian; 10-13-17 at 05:51 PM.