Let’s take a look at the history of the realm known as Ultramar. It’s not a world but an entire region of space ruled over by the Ultramarines!
Via our good friends at Lexicanum
Prior to the formation of Ultramar, the world of Macragge had maintained contact with its neighboring systems, its industries and peoples having largely survived the chaos of the Age of Strife. The world would be transformed however by Roboute Guilliman, becoming a prosperous and well-governed planet with trade routes that brought in raw materials and immigrants on a regular basis.
It was during this time that Guilliman was rediscovered by the Emperor, who named him Primarch of the Ultramarines Legion and moved their forward base to Macragge. Within a year a new training base was operational and began taking recruits not only from Macragge, but from the surrounding systems as well. This close relationship with the other worlds would form the basis of what would become the Ultramarines’ stellar empire.
During the Great Crusade Ultramar maintained a steady supply of new recruits for the Legion, such that the Ultramarines became one of the largest Legions ever. It, along with the Ultramarines themselves, was left largely unscathed by the Horus Heresy, though it was invaded by the Word Bearers resulting in the Battle of Calth.
In the aftermath of the battle the Word Bearers were able to erect the Ruinstorm, severing Ultramar from the rest of the Imperium. Guilliman became worried that the Imperium itself was lost and sought to have Ultramar become the heart of a new contingency empire: Imperium Secundus.
With the end of the Heresy, Ultramar was not attacked again until the War of the Beast when Ork forces landed on Quintarn and Prandium. Ultramar next came under attack in the First Tyrannic War with the invasion of Hive Fleet Behemoth. Though ultimately defeated, Behemoth would leave its mark, especially with the complete destruction of Prandium.
Ultramar would be threatened a third time when the Iron Warriors Warsmith Honsou assembled an enormous warband to invade Ultramar and seek revenge against Captain Uriel Ventris. While the Chaos forces would be beaten back, the War for Ultramar would see the greatest loss of life since the Tyranid invasion of Hive Fleet Behemoth.
