The Angel Exterminatus is the Gothic name for an immeasurably powerful creature of Eldar Mythology. In the Eldar’s tongue, it is called Maelsaha'eil Atherakhia, although no survivor of the Eldar Craftworlds would dare to say its name aloud, which means 'beautiful eagle from hell that brings the end of all things'. The Angel Exterminatus plays an important role in cycle of the House of Eldanesh, the Eldar's greatest hero and seems to have strong ties through its name and role to the god Kaela Mensha Khaine, of whom it is believed by Imperial mythologists to be some sort of physical manifestation -- an Avatar of the god's wrath. Though having vanished from the face of the universe long before the rise of the Imperium of Mankind, it is known that in the early 31st Millennium, two of the Traitor Primarchs -- Fulgrim of the Emperor's Children and Perturabo of the Iron Warriors, respectively, coveted the Angel Exterminatus' power and tried to unleash it against the Loyalist followers of the Emperor of Mankind. This attempt however, was spoiled by the valiant actions of the Shattered Legions survivors of the Iron Hands' Strike Cruiser Sisypheum.
According to myth, the Angel Exterminatus precedes the War in Heaven and thus belongs to the oldest myth of the Eldar Myth Cycles. The great houses of Eldanesh and Ulthanesh had successfully conquered the galaxy, but still longed for conquest. However, the galaxy was a barren and desolate place were nothing lived. In desperation, Eldanesh implored the goddess Isha -- who had given birth to the Eldar race -- to create some new races to wage war against and to subdue. Moved by this request Isha shed bitter tears, and from these tears, life sprang. Amongst these races created by the tears of the goddess were a race referred to as the Hresh-selain in the Eldar Lexicon -- a breed that survived in the darkest regions of space and that was rapidly conquered by the Eldar. But the king of the Hresh-selain harboured dark thoughts of retribution and soon began to plan a way to challenge the Eldar's supremacy. Rebuilding their armies in a distant dimension that was out of reach of the Eldar, the Hresh-selain, their king at their head,, struck unexpectedly against the Eldar. War soon engulfed the galaxy, leaving devastation in its path, the hordes of the Hresh-selain outnumbering the stars, who bitterly fought the Eldar. Soon the Eldar's losses numbered in the tens of thousands, with even their peerless abilities unable to make a dent in the billion-strong ranks of their enemy. The Eldar Gods wept at the fate of their children, and the Eldar were on the the brink of defeat, when once again Eldanesh turned to his mother for help. So moved was Isha that she took Eldanesh's plea to her brother and husband, the war-god Kaela Mensha Khaine. Isha asked of Khaine to stand and fight alongside the Eldar, but Khaine refused, having always been jealous of Asuryan's and Isha's brood. Bound to help her children no matter the cost, Isha struck a devil's bargain with Khaine, offering him what he had always coveted -- her own, once-virgin flesh. This swayed Khaine and he violated her flesh, his blood-tainted seed and Isha's sacred flesh giving birth to a fiery avatar. The avatar's bloody claws ripped open Isha's womb, her death-scream mingling with the Angel Exterminatus' birthcry.
With the Angel Exterminatus more than eager to join the fray, the war against the Hresh-selain soon turned in the Eldar's favour. The Angel Exterminatus was by far the most powerful of the Eldar demi-gods, casually wielding powers even the gods did not dare to unleash whilst knowing everything the gods themselves knew. While Ulthanesh shrieked from this new creature, Eldanesh welcomed this new ally even if its birth had brought the death of their own mother. Eldanesh came to relish the smell of blood on his hands, of fire in his nostrils and the sight of the corpses of the Hresh-selain being picked clean by carrion birds. His hatred for his enemies and his bloodlust knew no boundaries, as did his jealousy for the power that the Angel Exterminatus wielded. With the utter destruction of the Hresh-selain, their common goal achieved, Eldanesh gained the Angel's trust, asking it to forge new weapons of terrifying power -- weapons of whom it was said, their crafters' preferred to take their own lives rather than continue to exist in a galaxy that had spawned such terrible weapons. Even to a being as formidable as the Angel Exterminatus, the crafting of such magnificent weapons was a tiresome task. Upon completion of the weapons, it quickly sunk into a deep sleep but not before it had given Eldanesh what he had asked for. Finally holding the means of his ultimate victory in his hands, the veil of madness and bloodlust that had overcome Eldanesh finally parted and he realized with horror what he had been about to inflict to the galaxy.
Calling out for his brother Ulthanesh, the two kings of the Eldar asked mighty Asuryan for forgiveness and devised a plan to banish the Angel Exterminatus beyond space and time into the netherworlds. Sensing this turn of fate and the danger it was in, the Angel Exterminatus awoke and a new war broke out between the erstwhile allies. The conflict between the demi-god and the Eldar almost shattered reality itself. No one would give their enemy any form of quarter. No one knew how long the fight between Eldanesh, Ulthanesh and the Angel Exterminatus lasted, but in the end the twin-brothers could not overcome the Angel Exterminatus, and sank to their knees, utterly exhausted but defiant to their last breath. It was then that Asuryan himself intervened, shattering the already weakened fabric of reality and folding it around the Angel Exterminatus, thus imprisoning it in a hell of its own making. By decree of Asuryan, the seals on the Angel Exterminatus' prison were never to be broken while the uttering of its very name was soon considered a crime.
- Angel Exterminatus (Novel) by Graham McNeill, pp. 91-97