Endryd Haar, known as the "Riven Hound," was once a World Eater, part of the first intake of initiates created on Terra, long before the XIIth Legion came to be know as the War Hounds, and several decades before the rediscovery of their Primarch Angron upon the world of Nuceria. Endryd Haar would go on to renounce his former allegiance to his traitorous Legion when they declared for the Warmaster's cause during the early years of the Horus Heresy. He would go on to fight alongside other Loyalist forces leading his own Blackshield unit known as "The Fangs of the Emperor," fighting the Traitor forces across the width and breadth of the galaxy.
Endryd Haar and his three battle-brothers were part of the first initiate company of the XIIth Legion created in the Imperial Geneworks on Terra, long before his brethren had taken the name of the War Hounds, and many decades before they would become Angron's "eaters of worlds." Only he would go on to survive that company; a remnant of a lost age, a living weapon akin to his Legion brothers that came after and yet subtly different in his controlled savagery. Endryd Haar survived dire wounds and returned time and again from shadowed fates that have spelled the doom of all those around him as if he had been preserved by some unforeseen hand.
Haar and his command were believed long lost on-Crusade when his brethren cast in their lot with the Traitors. Endryd was driven to cold madness by the revelation of his Legion's betrayal when he returned to find the Imperium riven by civil war, and he cast off all traces of his Legion's insignia and honours and swore a Death Oath to atone for the XIIth Legion's crimes. Leading a Blackshield unit known as "The Fangs of the Emperor," Endryd Haar fought alongside the Loyalists as a field commander in the dark days before the Siege of Terra, accepting any mission -- whatever the odds of survival -- so long as in doing so he could spill the blood of the enemy. Haar's ultimate fate is unknown.
- The Horus Heresy - Book Six: Retribution by Alan Bligh, pg. 246-247