Ardium is the sole Imperial Hive World of the Macragge System, the heartland of the Realm of Ultramar, the famed Sector Imperialis governed by the Ultramarines Chapter. Once deemed inviolable, Ardium and its three subterranean Hive Cities have recently become part of the battlefields upon which both the Ultramar Campaign and the Plague Wars have been fought.
For much of its history, Ardium has stood in the shadow of its far more influential neighbor, Macragge, the ancestral homeworld of the Ultramarines. At the beginning of the 42nd Millennium, the Hive World came to the attention of the Forces of Chaos, which launched two separate invasions in the wake of the opening of the Great Rift. Ardium is still recovering from both these events.
The Ultramar Campaign
In the wake of the destruction of Cadia during the 13th Black Crusade, the Warmaster of Chaos, Abaddon the Despoiler, launched a sizeable portion of his fleet in pursuit of those Imperial troops and their allies that had escaped destruction by entering the Aeldari Webway. With Macragge itself recently assaulted by the troops of the Daemon Prince M'kar the Reborn during the Invasion of Ultramar, the other worlds of the Macragge System were mostly left to fend for themselves.
Ardium was invaded by the Bitter Sons, a splinter-faction of the Iron Warriors Traitor Legion that had sworn allegiance to the Despoiler. Despite heavy resistance, these Chaos Space Marines conquered one of Ardium's three principal hive cities, Magmaria, and immediately began fortifying its winding, subterranean tunnel networks for the expected counterattack. Awakened from his state of near-death on Macragge by the artifices of the Archmagos Dominus Belisarius Cawl and the power of the Aeldari god of the dead Ynnead, the Primarch Roboute Guilliman led the counter-invasion of Ardium after having repelled the won the Ultramar Campaign.
Desperately outnumbered by the 4th and 6th Companies of the Ultramarines, the Bitter Sons stubbornly fought on and although they died to the last man, they inflicted significant damage to both their opponents and Magmaria's infrastructure.
The Plague Wars
With Ardium and the whole Macragge System still reeling from the blow struck against it, the Daemon Primarch Mortarion of the Death Guard Traitor Legion launched his own attack against the Realm of Ultramar, an invasion later named the Plague Wars. Once again, Ardium was invaded by the followers of Chaos; this time, it was the disciples of the Plague Lord, Nurgle, that threatened its security.
While the Ultramarines aided in the planet's defence, they were powerless to prevent the Death Guard's Plague Marines from gaining a foothold upon Ardium. Guilliman firmly believed that the invasion of Ardium hid a terrible ploy, one that could threaten the outcome of the entire conflict. He sent Captain Felix and the newly founded 11th Company of the Ultramarines Chapter to liberate Ardium and foil whatever vile scheme Mortarion had concocted. Both Captain Felix and his warriors were Primaris Space Marines recently arrived in Ultramar who would soon prove their merit on the field of battle.
Captain Felix discovered an unholy device on Ardium that was a mix of xenos technology and Chaos sorcery created using the knowledge of the ancient alien artefact known as the Hand of Darkness by Mortarion himself. This technology slowly transformed Ardium's unaugmented human population into hideous creatures of Nurgle. Launching a daring mission that ended in the device's destruction, the 11th Company later gained the upper hand against their their foes and broke the Death Guard's hold on the planet.
- Dark Imperium (Novel) by Guy Haley, Chs. 18, 20
- Gathering Storm - Part Three - Rise of the Primarch (7th Edition), pp. 11, 41-47