Treyen is a Feral World within the Achilus Crusade's Orpheus Salient in the Jericho Reach. A largely temperate world, Treyen's population is centred primarily in the western hemisphere. The last Imperial census had the population at roughly 250,000 with an average technology level equivalent to that of a bronze age.
The primary star within the system is entering into its last stages of life, and as such solar flares are beginning to spread a blanket of radiation around the planet.
This stellar activity has a nasty habit of causing long-range vox transmissions to be unreliable at best, but it also seems to have an equal effect upon the communications of the Tyranids on-planet. Treyen was chosen as a suitable battleground by the leaders of the Orpheus Salient for these specific peculiarities.
When the Achilus Crusade first encountered the planet Treyen, it was found that the natives there worshipped their sun as a god. While this deity provided them with warmth, he could also bring terrible vengeance upon them when he was angered.
He would provide them with warmth, shelter, food, and protection, but if they ever angered Him or His chosen representative, He would strike them down with a furious anger the likes of which they had never seen before.
While the Departmento Munitorum was attempting to calculate what the planet's tithe grading would be, it was discovered by an adventurous group of Astra Militarum guardsmen that the large native beasts had a unique pelt that, when skinned, had the ability to change colouration with the lighting.
With this discovery, the planet's primary export was quickly changed from baseline foodstuffs to luxury goods. Rumour has it that General Derian Arkelius himself has his crusade dress uniform made from the hide. No one has seen this uniform, but it would fit his public persona.
After classifying and settling the planet, a small defence force was left behind, mainly to hold off any Orks tempted to raid this deep into the crusade's battleline. Much of their time on planet was spent with very little to do, as the native population was fairly tame and had nothing but love for the God-Emperor.
Occasionally there would be a small uprising by one of the tribes that had been displaced when the Astra Militarum claimed certain land as their headquarters, but savages with swords and axes do very little against trained Imperial troops with lasguns.
The Arrival of Hive Fleet Dagon
At the beginning of 815.M41, the small garrison received a rather abrupt wake-up call. Lord Commissar Vance Theren and Battlegroup Cerberus arrived in-force to stand in the way of a new xenos threat that was approaching.
When Theren landed, he exited his lander quickly and set up a command post inside the Astra Militarum headquarters for himself and the rest of the Triumvirate that commanded the Orpheus Salient.
Immediate orders were given to begin erecting defensible positions. Swarms of Mechanicus servitors skittered off of the lander in his wake and began stripping the compound of all materials needed to enhance the base's defence.
Over the next two solar weeks thirty-three full regiments of the Astra Militarum, almost the entirety of the Orpheus Salient complement, landed with the Scintillan 84th and General Derian Arkelius at their head.
Artillery pieces were quickly placed in defensible positions. Sentinels and snipers were sent to reconnoitre the surrounding terrain. Lord Commissar Theren barricaded himself in his command post with General Arkelius in an attempt to find a way to effectively hold the line at Treyen.
It wasn't until the fifteenth day that Theren received three communications. The first was from the Lord Admiral Anastasia Arkelius, saying that if the shadow being cast in the Warp was any indication, the Tyranid force -- which would later be designated Hive Fleet Dagon -- was massive. The second came from the Dark Sons Chapter of Space Marines. The 1st Company with its squads of Terminators would be arriving in three solar days.
The third was from his fleet Astropath. The Tyranids would arrive in only two solar days. When the hive fleet arrived, Lord Admiral Arkelius proved to any doubters exactly how she had achieved her position. Fully one-third of all Tyranid vessels were destroyed in the naval engagement above Treyen.
Her fleet was badly damaged, but she was still able to order her picket ships to provide close fire support for the upcoming battle. On the ground, despite the situation in orbit, the task ahead of the sixteen million guardsmen seemed hopeless.
What appeared to be a monsoon of living monstrosities began pouring out of the sky and wave upon wave of hungry xenos beasts rushed towards the Imperial battle lines. For twenty-four solar hours the entire planet of Treyen was a battlefield. Millions of guardsmen were killed, and even more were wounded.
Eighteen solar hours into the engagement, streaks of light came hurtling towards the planet's surface. Over the communication system came a message telling Lord Commissar Theren to order a full counterassault in exactly one solar hour. Attached to the message were the authorisation codes of the Deathwatch.
Precisely one hour later, the Tyranid horde fell into absolute disarray, and Lord Commissar Theren sent planet-wide orders to begin the counterassault. Five hours later, Lord Commissar Theren met up with the Deathwatch Kill-teams that had been inserted on the planet via Drop Pod.
These Kill-teams had destroyed the largest Tyranid bioforms, thus disrupting the creatures' ability to communicate with one another. With no enemy leadership in place, the Astra Militarum was able to take advantage of the Tyranids' confusion and drive them back into an open area where the fleet in orbit could bombard the xenos.
When compiling his after action report, General Arkelius noted that if the Tyranids had continued engaging the fleet in orbit, it would have been impossible for the Deathwatch to get close enough to be successful. By committing to the all-out attack too early, the Tyranids had cost themselves the battle.
When the Dark Sons arrived the next day, they were quickly able to hunt down and remove all obvious signs of Tyranid infestation. All three Astra Militarum commanders were acknowledged for their bravery and skill in handling what would normally have been an impossible situation.
The one drawback was that the attacking Tyranid force had now splintered apart. One part moved towards Hethgard, and the other appeared to be heading for Tryphon. The Deathwatch are currently keeping a very close eye on Treyen, as they need to make sure that the planet is thoroughly cleansed.
- Deathwatch: The Achilus Assault (RPG), pp. 53-54