Calth is a fortress world in the Ultramarines’ realm of Ultramar, known for its underground cities, orbital shipyards, and the catastrophic Word Bearers ambush that stripped its atmosphere during the Horus Heresy.
What calth is
Calth is an airless, rocky world in the Veridia System of Ultramar, the Ultramarines’ domain. Its surface is uninhabitable because of the deadly radiation from its blue sun, so its people live in vast cavern-cities below ground.
Despite that, it’s one of the most productive worlds in the region. Its orbital shipyards are among the best in the Imperium, and its underground factories keep Ultramar supplied.
- Location: Veridia System, Ultramar, Ultima Segmentum
- Class: Civilised World and Death World
- Affiliation: Imperium (Ultramarines)
- Population: lives in underground cavern-cities
Why calth matters
Calth’s shipyards and docking network made it a linchpin of Ultramar’s military power. During the Great Crusade, it was one of the most important worlds in the realm, and there were plans for it to rule its own fiefdom with a Tetrarch.
Its importance also made it a target. The Word Bearers chose Calth for their ambush precisely because crippling it would hurt the Ultramarines’ ability to fight back.
The battle of calth
After Horus became Warmaster, he ordered the Ultramarines and Word Bearers to muster at Calth for a campaign against the orks of the Ghaslakh xenohold. It was a trap. The Word Bearers, already corrupted by Chaos, launched a surprise attack on the Loyalist forces there.
The Ultramarines and their allies eventually won, but millions of civilians died. The attack permanently stripped Calth’s atmosphere and poisoned the sun’s radiation, making the surface lethal without shelter. From then on, almost everyone lived in the cavern-cities.
Calth after the heresy
Calth endured. It fought in the War of the Beast about 1,500 years later, and it was caught up in the tyranid invasion of Ultramar when Hive Fleet Behemoth arrived.
It’s still a key Ultramarines world today, a fortified shipyard hub that helps keep Ultramar running.
Key facts
- Calth is an airless Death World in the Veridia System, Ultramar.
- Its population lives in underground cavern-cities due to the deadly blue sun.
- Its orbital shipyards are among the best in the Imperium.
- The Word Bearers ambushed the Ultramarines there during the Horus Heresy.
- The attack stripped Calth’s atmosphere and killed millions of civilians.
- Calth later fought in the War of the Beast and against Hive Fleet Behemoth.
FAQ
What is Calth in Warhammer 40k?
Calth is an airless fortress world in Ultramar, home to underground cities and famous orbital shipyards. It’s part of the Ultramarines’ realm and played a major role in the Horus Heresy.
Why is Calth important?
Its shipyards and docking network made it a military hub for Ultramar. It was also the site of a devastating Word Bearers ambush during the Horus Heresy.
What happened at the Battle of Calth?
The Word Bearers ambushed the Ultramarines and other Loyalists at Calth early in the Horus Heresy. The Ultramarines won, but millions died and the planet’s atmosphere was destroyed.
Can you live on the surface of Calth?
No. After the Heresy, the surface is lethal due to radiation from the blue sun. People live in underground cavern-cities, with only a few shielded surface sites.