It was personally crafted and presented to him as a gift by the Emperor of Mankind when he was named as the Imperial Warmaster and charged with leading the Great Crusade as the Emperor's proxy following the Ullanor Crusade.
The Horus Heresy
Worldbreaker is said to have been created by the hand of the Emperor Himself and presented as a gift to His favoured son during the Triumph of Ullanor in 000.M30, the same day Horus was raised to become the Warmaster and the Emperor announced that He was returning to Terra. This massive weapon was easily the size of a mortal man marked out with the Imperial Aquila at the base of its grip.
As well as being a weapon capable of shattering armoured ceramite, it was also a signifier of Horus' rank as the Warmaster of the Imperium. Horus was the first and foremost of the Primarchs and master of the Luna Wolves Space Marine Legion and the most favoured son of the Emperor. Horus Lupercal was capable of bearing the massive Power Maul one-handed, but the immense mace was too cumbersome for any of the other Astartes to wield it with any grace. A bludgeon of darkened metal, its spiked head alone was the size of a Space Marine warrior's entire armoured torso.
When Horus fell on Davin's moon and became corrupted by the Ruinous Powers, he unleashed a devastating civil war known as the Horus Heresy, and ushered in the so-called "Age of Darkness." His formidable weapon lived up to its name, as he broke world after world, until finally, his nine-year-long rebellion culminated in the final battle at the Siege of Terra.
In the final hours of the conflict, Horus allowed the Emperor and an entourage of Loyalists to teleport directly to his flagship, the Vengeful Spirit. He went on to use Worldbreaker to kill his brother Sanguinius, the angelic Primarch of the Blood Angels Legion, who spurned Horus' invitation to join his dark cause. When the Emperor came upon the horrific scene, Horus stood over the broken and bleeding body of his brother. Father and son then confronted one another, one final time. In the end, the Arch-Traitor was cast down, but not before he managed to mortally wound the Emperor.
The Legion War
It was First Captain Ezekyle Abaddon who found the fallen Warmaster. He took up the lifeless body of his father as well as his weapons, and abandoned the conquest of Terra. Instead he retreated with the Sons of Horus Legion and all of their remaining assets, Legion slaves and starships, blazing a trail across the stars to the forbidden realm of the Eye of Terror. The Sons of Horus managed to reach the Eye of Terror with the bloodied survivors of the Great Scouring, but the once mighty XVI Legion was reduced to a fraction of its former size.
Led by only a few remaining captains, the Legion struggled with its loyalty to their fallen Primarch and the cold reality of their defeat at the hands of the Emperor and His lackeys. Bereft of their glorious Primarch, the Legion floundered, and in desperation turned to each of the Chaos Gods in their search for renewed power, inviting daemonic possession and the ever more costly blessings of the Warp. All the while, the Legion suffered the jealous attacks of their former allies.
Ezekyle Abaddon abandoned the Legion; broken by the death of Horus and sick of war, he wandered alone into the Eye of Terror. Taking his Legion's massive flagship, the Vengeful Spirit, Abaddon left his brothers behind and plummeted into the furthest reaches of the Eye. Meanwhile, the Sons of Horus carried the body of their Primarch, preserved in stasis, further into the Eye, ignoring the wars between the other Traitor Legions that raged around them.
On the Daemon World of Maeleum, a graveyard world of steel and rust, the Sons of Horus raised a fortress, the Lupercalios, fashioning a mighty citadel from the wrecks of decaying vessels lost to the Warp with the aid of the thousands of slaves they had taken from the worlds of the Imperium. But in a sudden assault, the remnants of the debased Emperor's Children, having grown vastly in power after firmly cementing their terrible pacts with Slaanesh, easily smashed their way through the defences of Maeleum and into the central chambers of the Sons of Horus' stronghold.
They stole the body of the slain Primarch from the heart of its tomb and spirited it away with the purpose of handing it over to the dark Apothecary Fabius Bile who intended to clone it in order to create a new and still greater Warmaster of Chaos to restore the Traitor Legions' unity and fortunes.
Following the sundering of the XVI Legion and the destruction of the Primarch's monument in the Lupercalios on Maeleum, Falkus Kibre, the former Captain of the Sons of Horus 1st Company's Terminator Squad known as the "Widowmakers" of the elite Justaerin, eventually sought out with his allies Iskandar Khayon and Lheorvine Ukris the whereabouts of the missing First Captain Abaddon. The salvation of the Sons of Horus came when Ezekyle Abaddon returned from his Dark Pilgrimage. It was in that moment that he saw, with cold clarity, that it was Horus' failure that had led the Legion here, to them tearing each other apart in the blood-soaked ruins of Maeleum.
Abaddon swore that he would succeed where Horus had failed in overthrowing the "Corpse-Emperor" and proclaimed himself the new Warmaster of Chaos. Finally, sickened by how far the Legion had fallen, he stalked through the ruins hunting down his fellow captains, cooling his rage with their final screams. In the end, Abaddon alone remained of the Legion's officers, demanding obedience from his brothers.
With his Legion brought to heel, Abaddon turned his attention to the clones of Horus; he commanded his warriors to extinguish all trace of their former Primarch and free themselves from his shadow. He then personally led an attack on the Emperor's Children to destroy the body of Horus and the other Primarch clones created by Bile. After destroying the aberrant creations of the Clone Lord, Abaddon faced a perfect clone of the lost Warmaster that had already been completed by Bile.
During this final confrontation, the Clone-Horus swung Worldbreaker at his former First Captain in an attempt to kill him. But Abaddon caught the immense maul in the grip of the Talon of Horus, the Warmaster's former signature Lightning Claw that was still stained with the blood of a god and His angel. Abaddon closed his fist, and Worldbreaker was broken -- shattered into so many shards of scrap metal. Abaddon rammed all five claws deeply into the Clone-Horus's chest and out his back. He then fired the Talon 's built-in Combi-Bolter, killing the cloned Warmaster, and in so doing, ushered in a dark new age for the XVI Legion, no longer the Sons of Horus, but Abaddon's own Black Legion.
- The Horus Heresy Book One: Betrayal (Forge World Series) by Alan Bligh, pg. 247
- The Talon of Horus (Novel) by Aaron Dembski-Boweden