The Betrayal at Calth is one of the defining moments of the Horus Heresy. It’s the surprise attack by the Word Bearers on the Ultramarines’ home world of Calth, and it’s been covered in novels, a board game, and even a video game.
What happened at Calth?
During the Horus Heresy, the Word Bearers legion turned on their loyalist cousins, the Ultramarines. They launched a devastating assault on Calth, a key Ultramarines world. The attack was part of a wider plan to cripple the Ultramarines and cut off reinforcements to Terra.
The Word Bearers struck without warning, using subterfuge and Chaos corruption. The Ultramarines were caught off guard, and the battle quickly descended into a brutal underground war in the caverns beneath Calth’s surface.
The betrayal was personal. The two legions had once been close, and the Word Bearers’ primarch, Lorgar, had been chastised by the Ultramarines’ primarch, Roboute Guilliman. The attack was partly revenge.
Why does it matter?
Calth showed that the Horus Heresy wasn’t just a civil war among Space Marines; it was a war of ideology and faith. The Word Bearers had embraced Chaos, and their attack on Calth was a religious crusade as much as a military one.
The battle also tied down a significant portion of the Ultramarines legion, preventing them from joining the defence of Terra. Though the Ultramarines eventually won, the cost was high, and the planet was left in ruins.
For fans, Calth is a fan-favourite setting because it pits two iconic legions against each other in a desperate, claustrophobic conflict. It’s been explored in the novel Know No Fear by Dan Abnett, the anthology Betrayal at Calth, and the board game of the same name.
The board game
Games Workshop released The Horus Heresy: Betrayal at Calth in November 2015. It’s a stand-alone miniature wargame for two players, focusing on squad-based combat in the caverns under Calth.
The box came packed with miniatures: a Space Marine Captain in Terminator armour, a Contemptor Dreadnought, five Cataphractii Terminators, and thirty Legion Tactical Marines in MK4 armour. You also got four double-sided boards, a 48-page sourcebook, command cards, dice, and markers.
The game was followed by The Horus Heresy: Burning of Prospero, which used a similar format. It’s a great entry point for anyone curious about Horus Heresy gaming.
The anthology and other stories
Black Library published an anthology also called Betrayal at Calth in November 2015. It collects two novellas: The Honoured by Rob Sanders and The Unburdened by David Annandale.
The Honoured follows the Ultramarines, while The Unburdened shows the Word Bearers’ perspective. Together they cover the Underworld War on Calth, focusing on the rivalry between Kurtha Sedd of the Word Bearers and Steloc Aethon of the Ultramarines.
If you want the full picture, Know No Fear is the main novel, and Mark of Calth is a follow-up anthology. There’s also an audio drama called Censure.
The video game
A turn-based tactical game called The Horus Heresy: Betrayal at Calth was released on Steam. It lets you command Ultramarines against the Word Bearers, and it supports both PC and VR.
The game had a rocky development, with long gaps between updates. As of early 2025, it’s still in Early Access, and the developers have said they’re working on a final release. It’s not required to have a VR headset to play.
Key facts
- The Betrayal at Calth was a surprise attack by the Word Bearers on the Ultramarines during the Horus Heresy.
- It took place on the Ultramarines world of Calth and led to a brutal underground war.
- Games Workshop released a stand-alone board game called The Horus Heresy: Betrayal at Calth in November 2015.
- Black Library published an anthology of the same name, featuring novellas by Rob Sanders and David Annandale.
- A turn-based video game adaptation is available on Steam, with VR support.
FAQ
What is the Betrayal at Calth?
It’s the surprise attack by the Word Bearers on the Ultramarines at Calth during the Horus Heresy. It’s also the name of a board game, an anthology, and a video game based on that battle.
Why did the Word Bearers attack Calth?
The Word Bearers had turned to Chaos and wanted to cripple the Ultramarines, who were loyal to the Emperor. It was also revenge for the Ultramarines’ role in humiliating their primarch, Lorgar.
Is the Betrayal at Calth board game still available?
It’s out of print from Games Workshop, but you can find copies second-hand. The miniatures are still usable in games of Warhammer: The Horus Heresy.