"Be careful of what you slay, lest the Warp rise up to the Deep and becalm you amidst your sins" -The Ancient Mariner
Such is the Mariners' way of referring to the incident that caused the death of his entire company. Once a captain when the Star Krakens were young, he was part of a the company aboard "The Albatross" searching for signs of tyranids amongst the world of Helvellyn. The Mariner was known for having a prefernce for ship to ship combat, and boarding actions, a prelude to the temerament of the chapter. He was known so widely by his brothers that he was named The Mariner, and was thought to be on the path to being a champion for the Krakens. The mission that doomed him was an action taken when it was discovered that warp travel was being impeded, one of the first signs of tyranid presence created by the Shadow in the Warp of the tyranid Hive-Mind. This was not helped by the fact that the Monks of the Words had become more distant recently, a point of suspicion that would later twist the mind of the Mariner
It was the rash actions of the Mariner, not so ancient at the time, that doomed the world and his company. Helvellyn was a shrine world, dedicated to the Saint of the Words, who had brought the Imperial Creed to the sector through words along where might had not brought about worship of the Emperor. What the captain found on the shrine world was not tyranids, but heresy. Even as the marines of the "The Albatross" walked across Helvellyn, they were put off by the strange shrine-world, and its inhabitatns pension for the light and airy despite the darkness of the Deep in which their world sat.
Within the greatest monument to the Saint of the Words, a great mount covered with flowers that girdled tombs, covered in reverential poetry of heroic deeds, and topped by a palatial shrine, the Captain Mariner found a foul ritual taking place. Inside the tomb of Ecclesiarch Kueblye, cultists had made their lair, servants of the Prince of Pleasure. They had desecrated the tomb of the former ecclesiarch, doing usnpeakable deeds with the remains, slicing their limbs with thin knives to dribble blood on the ground in patterns that looked like disgusting poetry. The foul act led by none other than a foul xeno, a monstrous thing with a gaping maw, a tangle of tentacles and enormous, black eyes. The Mariner took battle to the cultists, and slew them all with the advantage of surprise. With its dying breath, it taunted the Mariner, for it claimed that his servants had already infiltrated the whole world, and that soon the songs of slaanesh would be spoken by every mouth on Helvellyn. From the ritual that had been performed came daemons that tore apart the cultists, and chased the Captain and his squad out as the beasts slew the monks, and hundreds of cultists broken up tombs of dead Star Krakens buried on the mount to create more rituals.
Incensed by the presence of a xeno in the heart of a shrine world, and already suspicious of the very monks his chapter had once called friends, the Mariner returned his strike cruiser and discovered that the demons weren't dissapting, but were increasing in number, a sure sign that the portal the warp had not been closed. Even as the Mariner watched, some of the daemons took to the sky, screaming disks that brought down a thunderhawk attempting evacuate a squad of Krakens.
The Mariner-Captain felt he had no choice, and ordered an orbital strike on the mount and what the captain referred to as a "pleasure palace" in his rage. It all seemed so clear to him. The turbulence, the strangeness of the shrineworld, and presence of the grendel. He was sure of himself, and thought that destroying the mount and palace would end the heart of the heresy that had taken root on Helvellyn, and allow a full campaign to reclaim the world
Only in the aftermath of this strike did the captain learn of what he had done. His actions had fueled a great sacrifice for the true ritual of chaos, one that made the warp around Helvellyn more turbulent, and let loose warpspawn from the ruins of the mount. Horrendous beasts spewed out of what was now a measureless cavern, a river to the domain of daemons. Horrified by what he had done, the Mariner ordered his cruiser and its escort to flee into the warp, and gain the aid of the rest of their chapter to cleanse Helvellyn. This did not go well, for as soon as "The Albatross" and its escorts entered the warp, they were beset by daemons. Though "The Albatross" would make the trip, every soul aboard, save for its captain, would perish as they fought the consequences of the captains sin.
The Mariner crawled to the bridge, and let his brothers know what he had done, and that he needed help to rectify his actions. His only answer was the Chapter Master himself, saying he could not allow the chapter, so youthful and full of vigor, to be tainted by actions of the captain. Roam the stars, was the Chapter Masters command for the Mariner, till he died. And so the Mariner set off, using the few servitors aboard to guide the ship, wracked as it was the taint of the warp. Now "The Albatross" has followed the Krakens to the deep, and haunts them, condemning them for betraying their brother, and in penitence of its masters own sin.