The result were hideous creatures utterly consumed by the Red Thirst that were exploited by the machinations of Fabius Bile in an attempt to distract the Blood Angels while he stole a sample of the Primarch Sanguinius' blood to further his own dark research into "improving" upon the Emperor's original work.
In the aftermath of Arkio's manipulation by the forces of Chaos and the resulting schism that nearly tore the Blood Angels Chapter apart, the sons of Sanguinius teetered on the brink of extinction. Lord Commander Dante summoned all of the Blood Angels' Successor Chapters for a conclave on Baal.
The Apothecaria was working on a radical new procedure to replenish the ranks of the Blood Angels in one swift stroke: the long-lost art of cloning or replicae as it was called in High Gothic, similar to the techniques that the Raven Guard had once attempted to rebuild their Legion following the tragic events of the Drop Site Massacre during the Horus Heresy.
The experiments proved fruitless and Commander Dante ordered a halt to further attempts. However, the Sanguinary Priest Caecus disobeyed the order and secretly continued the experiments, prodded by an agent of the vile former Emperor's Children Apothecary Fabius Bile.
Caecus was tricked into bringing Bile, who was disguised as a well-known Adeptus Mechanicus Magos Biologis named "Haran Serpens," to Baal, mistakenly thinking the renegade "Magos" would help in cloning Space Marines to increase the Blood Angels' declining numbers.
With the secret assistance of Bile, Caecus created a stable clone from his own gene-seed, which he christened the "Bloodchild." At first the cloning project was believed to be a great success, but when the clone was matched in mock combat against the combined might of Sergeant Rafen and the Flesh Tearers Sergeant Noxx, the "Bloodchild" ultimately succumbed to the Red Thirst within a matter of only solar minutes. After the Bloodchild drank blood from a wound it inflicted on one of its foes, it rapidly transformed into a monstrous, vampiric mutant.
Dante ordered Caecus to destroy his remaining prototype clones, but when the Sanguinary Priest returned to his laboratory, he discovered that Serpens had already released them to rampage through the Blood Angels' fortress-monastery. Bile, possessed of a dark sense of humour, had christened the creatures "Bloodfiends" and was already preparing to flee Baal as the blood-drinking mutants ran wild, though not before he revealed his true identity to a shocked Caecus. The Clonelord also stole a glass vial which contained a sample of the distilled blood of the Primarch Sanguinius that had been kept in the Chapter artefact known as the Red Grail.
The Bloodfiends attacked the Blood Angels across the fortress-monastery complex, proving stronger and more vicious than the Astartes. Despite their feral nature, they soon began to scavenge armour and weapons from their blood-drained victims, which enhanced their combat abilities. Overwhelmed by the desperate need of the Red Thirst, the Bloodfiends, led by the so-called "alpha clone," penetrated the Sanguinary Chapel where the Red Grail and its precious sample of Primarch blood was kept.
The alpha clone sacrilegiously drained the Red Grail dry, which only made the creature's thirst for blood worse. Dante himself led the assault of the Blood Angels to reclaim the Sanguinary Chapel and the Sepulchre where the Primarch's body had lain in state for over ten thousand standard years. It was Rafen who slew the alpha clone within a hand's width of the Primarch's sarcophagus. The Blood Angels scoured the Sepulchre clean of the remaining Bloodfiends, finally eliminating the taint of Chaos from their midst.
Anatomy and Physiology
After being decanted, Bloodfiends were feral, primitive humanoids. As they drank more blood or consumed blood of greater potency than normal like that of an Astartes or a Primarch, they grew in strength and intelligence. After a Bloodfiend made a kill, it drained the body of its victim dry of all blood and other vital fluids.
After tasting blood, Bloodfiends mutated into dark, reddish-brown humanoids whose bodily density was great enough to withstand even multiple shots from a Space Marine Bolter. They possessed extra joints in their limbs and extremities and their mouths were filled with rows of sharp fangs that were used to puncture the skin of their victims and devour their blood. They rapidly developed bat-like membranes between their limbs that allowed them to gracefully glide to the ground from a great height.
It is unknown whether the Bloodfiends' rapid mutation was the result of Fabius Bile's deliberate sabotage or Caecus' ignorance of proper replicae cloning procedures for Astartes. Similar Space Marine cloning experiments by the Raven Guard during the Horus Heresy had led to the birth of mutant abominations like the Bloodfiends, though those efforts were also sabotaged by the machinations of the Alpha Legion.
At the same time, when Caecus discovered Bile's true identity, the Clonelord mocked his poor attempts at genetic engineering. Bile noted that he had been experimenting with Astartes genetics for ten thousand Terran years and Caecus' crude attempts had never had even a chance of success.
- Red Fury (Novel) by James Swallow