"If Humanity will not unite, it will be destroyed. If the price of unity is a billion lives, or ten billion, or a number as like to the grains of sand on the shore, then we must pay it. If we do not, if we are not united, then we stand weak and divided against the remorseless enemies of an apocalyptic age."
- —Cardinal Salazar Scrone, Sermon on Mount Amalath
The Vanir System is a star system of Human-inhabited worlds that is the site of a frantic battle between the Imperial forces of the Achilus Crusade and rebels loyal to the self-proclaimed Monarchs of the Kingdom of Vanir. It is found within the Orpheus Salient of the Jericho Reach.
The desperate forces of Battlegroup Icarus, having withdrawn after taking massive casualties from the advancing Tyranids, are now locked in a race to overcome the rebel forces that have taken possession of the Vanir System.
The Imperials know that time is short and that the only chance of surviving the oncoming tide of Tyranids is to renew the system's defences before the Tyranids come to destroy Vanir and lay waste to Loyalist and rebel alike.
During the Jericho Reach's "Age of Shadow," Vanir was the seat of the false "Kingdom of Vanir," a petty empire held together by the military power of its Monarchs. A small fleet of Warp-capable starships kept the closely-situated worlds of the kingdom together through fear of orbital bombardment and the deployment of the Monarchs' dreaded gene-enhanced Crown Guard.
When the crusade forces reached the worlds of the kingdom, a solar year-long battle ensued that was ended with a highly dangerous attack on Vanir itself by Adeptus Astartes of the Storm Wardens Chapter and the bloody defeat of the Crown Guard.
Many of the Monarchs fled before the wrath of the Imperium as Vanir was forced brutally into Compliance; some were hunted down and executed by being crowned with molten gold, but a few escaped into hiding. Later, communications were broken by the Shadow in the Warp cast before the advancing Hive Fleet Dagon.
This threw the Imperials into disarray and the surviving Monarchs, long plotting the violent overthrow of Imperial rule, seized the opportunity to spark off a revolt.
This rebellion lifted the yoke of Imperial rule once more, bringing down the occupying government in a bloodbath of a civil war. The Monarchs regained control of Vanir.
Ragged elements of Battlegroup Icarus attempted to regroup at Vanir after fighting the invading Tyranids along the salient. However, they were attacked as they entered the system and found themselves fighting a war against the resurgent forces of the Monarchs.
In desperate need of the resources of the Vanir System, the battlegroup had no choice but to fight at a crippling disadvantage -- their fleet-based firepower muzzled by the fact that they could not risk destroying the planetary infrastructure or defences that they would need when the Tyranids arrived.
With time running short, Vanir remains a world divided and facing annihilation.
- Deathwatch: Core Rulebook (RPG), pg. 344