Of all the mighty weapons in Warhammer 40,000, none has caused as much damage as the Anathame. It was this Chaos-tainted blade of xenos origin that brought Warmaster Horus to the brink of death, and opened his soul up to the taint of Chaos. Even before the end of the Horus Heresy it was consigned to myth, but recent story developments have seen the blade re-emerge once more in the 41st millennium.
This article contains spoilers for various Horus Heresy books and the Dawn of Fire Warhammer 40k novel series.
The origins of the Anathame
The Anathame was created by the xenos species known as the Kinebrach, though when is unclear. Whatever their people's history with the powers of Chaos - or Kaos, as they called it - had been, it was far behind them when they were encountered by the Imperium's 63rd expeditionary fleet, close to the end of the Great Crusade. The Kinebrach had fully integrated into a multi-species society known as the Interex, where they coexisted peacefully with humans - in stark contrast to the xenocidal Imperium.
Erebus and the Anathame
Warmaster Horus Lupercal led a diplomatic mission among the Interex in the hopes of brokering a lasting treatise with this advanced civilization. Unknown to him, Erebus, Chaplain of the Word Bearers legion, had his own objectives. He broke into the Interex' Hall of Devices, a museum dedicated to armaments, and stole the Anathame - destroying any hope of peace in the process.
Erebus then transferred the Anathame to an ally, the seditious governor of the planet Davin, a man named Eugen Temba. Incensed when he learned of Temba's plans to secede from the Imperium, Warmaster Horus personally led the assault against Temba's forces stationed on the moon of Davin. He encountered Temba rife with the unclean power of the Chaos god Nurgle. Temba fought with supernatural might against the Warmaster, but ultimately fell to the Primarch. He inflicted just one shallow cut on the Warmaster using the Anathame - but this was enough to bring the demigod to the brink of death.
This opened the way for the next stage of Erebus' scheme, transferring Horus to the serpent lodge on Davin and exposing him to the powers of Chaos - but he was not done with the Anathame.
At the command of the Word Bearer's primarch Lorgar, Erebus was supposed to transfer the weapon on to Kor Phaeron, the Word Bearer's Dark Cardinal and Master of the Faith, to aid him in the coming conflict with the Ultramarines at Calth. Instead, Erebus divided the blade into eight parts, gifting each to a different leader within his Legion who was to take a role in the upcoming war.
From there, the history of the blade becomes muddied. At Calth, Kor Phaeron duelled Roboute Guilliman and succeeded in stabbing him with his shard, but - perhaps not wishing to be outdone by Erebus' achievements with Horus - he attempted to convert Guilliman, rather than kill him, and he failed. Phaeron fled into the warp with his blade and arrived at the daemon world Sicarus in the Eye of Terra, claiming it for his legion. Erebus retained his shard, and at the very end of the Heresy he stood at Abaddon's side as he fled Terra.
The Anathame reforged
Following the fall of Cadia and the opening of the Great Rift, many forces began to move within the Imperium of Man. Pulling the strings of many Chaos operations was the Hand of Abaddon, a council of sorcerers, Dark Apostles, Dark Mechanicum forge lords, and more. These agents set aside their mutual loathing to work towards a single goal - the reforging of the Anathame.
The components of the blade had been scattered throughout the galaxy. Kor Phaeron retained his shard, gifting it to the sorcerer Tenebrus to advance other schemes in the war against the Imperium. Graeyl Heryk, a renegade Space Marine of the Red Corsairs, retrieved a shard from a bound daemon blade kept by the Black Templars of the Ironhold Protectorate. Another shard was in Inquisition custody and destined to rendezvous with Fleet Secundus of the Indomitus Crusade, but Tenebrus' sorcery was able to redirect it into the clutches of the Hand.
The last act of the Hand was to begin the ritual that would reforge the Anathame, infusing the blade into the body and soul of Graeyl Heryk. This done, the members of the Hand fell to their old differences and returned to internecine fighting. Tenebrus' apprentice Tharador Yheng emerged as the victor, shepherded by the mysterious entity known as Augury.
To complete the reforging of the blade, Heryk underwent a pilgrimage into the warp, a journey of sacrifice and trials. At last he comes to the realm of Vashtorr the Arkifane, independent daemon of artifice. The Anathame was reforged, the weapon now known as the Culler.
But Heryk was betrayed - Augury promised that he would carry the blade, but not that he would wield it. Ambushed by Tharador Yheng, Heryk was defeated and Culler passed to her, becoming the talon of the new Hand of Abaddon. But Heryk was not so easily slain, and he does not plan to relinquish his prize.
Sources
The Anathame is stolen in Horus Rising, and used on the Warmaster in False Gods. It's shattered into pieces in Shards of Erebus, a short story in the collection Mark of Calth. The process of collecting the shards of the Anathame occurs across enough of the Dawn of Fire series that you may as well read the whole thing, and the blade is reforged in the penultimate book of the series, Hand of Abaddon.
What does Anathame mean?
The word Anathame is a pun or play on words. It's derived from the words 'athame' - a short-bladed knife used in ritual magic practises - and 'anathema', meaning 'an accursed thing' or even 'something excommunicated from the Church'.
Other athames in Warhammer 40k
The Word Bearers make extensive use of athame ritual daggers, which can get very confusing, since the shards of the Anathame can also be (accurately) described as athames. One very important athame that is easy to confuse with a shard of the Anathame enters the story in the Horus Heresy novel Betrayal at Calth. The Perpetual Ollanius Persson retrieves a stone athame from a Word Bearer's cultist, and uses it to cut a hole between worlds, effecting an escape for him and his companions.
This blade travels with Persson all the way to Terra during the final days of the Siege - we learn about its history in The End and the Death part one. The blade is truly ancient, made from the stone used in the first murder. This makes it the physical counterpart to the daemon blade Drach N'yen, or End of Empires, the psychic resonance of the same act (something that's described in the book Master of Mankind).
In The End and the Death part two, the stone athame is shattered by the sorcery of Erebus. Later it is reforged - seemingly unnoticed - by the Emperor, in the great burst of power that accompanies the moment he relinquishes the mantle of the Dark King. Ultimately, it is this blade that the Emperor uses to stab Horus Lupercal and slay him in the End and the Death part three. This is the last of eight lives the blade has taken, and it crumbles to dust.
If you're a fan of cursed blades (reading about, painting, duelling with), you'll definitely fit in in the Wargamer Discord community. Come and say hi!
If you crave deep lore, and Wargamer's Warhammer 40k faction guides aren't enough, may I suggest this article asking "is the Emperor an idiot?"; and this article on the Terminus Decree.