Serpent Lodge

The Temple of the Serpent Lodge, known also also as the Delphos, was a fane on the world of Davin, which was secretly temple devoted to Chaos Undivided, where the Warmaster Horus was treated for injuries incurred while quashing a rebellious Chaotic force on Davin's moon during the latter days of the Great Crusade in the 31st Millennium. With the Warmaster on the threshold of death, his senior commanders were desperate to save their Primarch's life and allowed him to be given to the Davinite Priests who were in truth a Chaos Cult, and using sorcery, which was outlawed by the Emperor, the cultists managed to warp the mind of the Warmaster against the Emperor by playing on the seed of jealousy and resentment that he felt for his father after the Emperor had left the Great Crusade behind to return to Terra. Magnus, the Primarch of the Thousand Sons Legion, attempted to unmask Erebus and reveal his manipulations, but Horus' corruption by the Ruinous Powers could not be stopped as he gave in to his feelings of jealousy, egotism and bitterness. Once Horus recovered, he schemed with the Astartes of the Luna Wolves' warrior lodge to overthrow the Emperor and purge the Legion of its remaining Loyalists, bringing about the great galactic civil war known as the Horus Heresy.

History
Some six standard decades prior to the events of the Horus Heresy, the XVI Legion, already called the Luna Wolves, in collaboration with the XVII Legion, then known as the Imperial Heralds but later renamed the Word Bearers, had undertaken the Imperial Compliance of the Feral World of Davin, codified as Sixty-Three Eight (63-8). A Feral World, Davin had been controlled by a remarkable warrior caste, whose savage nobility had won the respect of the Astartes sent to pacify their raging and bloody feuds. The Davinite warriors had ruled their world through a complex structure of warrior lodges, which were quasi-religious societies that had venerated various local predators as totem animals. By cultural osmosis, the lodge practices were quietly absorbed, emulated and modified by the XVI and XVII Legions to match their own internal Legion culture.

When Davin was brought to Imperial Compliance, the nomadic hunters of Davin had first taught the Luna Wolves of their tradition of creating a lodge for their tribe's warriors. The warrior lodge created by the Luna Wolves in imitation of this Davinite custom was a way for members of all martial classes to meet as friends, where captains could speak freely to their line Astartes, and vice-versa. The Luna Wolves embraced this custom, however it was kept as a secret, non-official gathering because of official Imperial disapproval until after the 200th year of the Great Crusade when Captain Garviel Loken discovered its existence within the heart of the Legion. At the start of the Horus Heresy, the lodge began to make decisions that would have far-reaching consequences throughout the Legion.

Officially, there were no warrior lodges, or any other kind of fraternities, within the Legiones Astartes. It was common knowledge that the Emperor frowned on such institutions, claiming they were dangerously close to cults, and only a step away from the illegal cult, the Lectitio Divinitatus, that supported the notion of the Emperor as the one, true God of Mankind. The Emperor openly and publicly refuted his alleged divinity and banned religious worship in his empire, and demanded that his subjects accept the "Imperial Truth"-- that science, reason and logic alone presented the tools required to create a better human future. There were many of those amongst the Space Marine Legions, especially those Astartes of Terran origin, who were openly opposed to the practise of the lodges, for to them it felt wrong, if nothing else, in that their practises were deliberately kept secret from other members of the Legion and were thus a form of deceit, since they knew the Emperor who have greatly disapproved of the custom. Many of those Space Marine officers within the Legions who disapproved of such clandestine activities let it be known to those Astartes that served under their command that they should have nothing to do with these lodges.

The creation of a warrior lodge within the Luna Wolves and many other Space Marine Legions based on the lodges used by the savage warriors of Davin had been another ploy of the Word Bearers' First Chaplain Erebus, one of the primary architects of the Horus Heresy, to infiltrate and corrupt the Legiones Astartes into turning against the Emperor. However, not all Astartes of the Legions joined the lodges, as many saw them as a direct violation of the Emperor's desires that all Space Marines dedicate themselves to truth and openness. The lodges would prove to be the primary purveyors of corruption in those Legions which turned Traitor in the days immediately preceding the start of the Horus Heresy and those Astartes within those Legions which lodges which held themselves aloof from those secretive organisations were marked as Imperial Loyalists who would later be betrayed during the Istvaan III Atrocity.

The Warmaster Falls
In the final days of the Great Crusade, during a battle against Chaos-spawned undead on Davin's moon, whose Planetary Governor, Eugen Temba, had been corrupted by the forces of the Chaos God Nurgle, Horus was poisoned by a xenos blade dedicated to Nurgle known as a Kinebrach Anathame. The blade had been stolen from the human civilisation of the Interex by the Word Bearers' First Chaplain Erebus after Horus and the Luna Wolves of the 63rd Expeditionary Fleet had made a disastrous first contact with them. In the course of that battle against Temba, the potent living metal of the Chaos blade wielded by the plague-infused monstrosity left Horus with a bleeding, toxic wound in his shoulder that his Legion's Apothecaries could not heal despite all the advanced technology available to them. Seeing his chance to further the nefarious designs of the Ruinous Powers, Erebus next persuaded the Luna Wolves' warrior lodge to allow a group of Davinite shamans -- Chaos Cultists all -- located on the surface of Davin at the Temple of the Serpent Lodge to heal him. The Luna Wolves, besides themselves with grief and the fear that their beloved Primarch would die, agreed to the suggestion, despite its direct violation of the creeds of the Imperial Truth.

With the Warmaster on the threshold of death, neither religion nor its denial seemed very significant any more. It was the only hope the Luna Wolves had left and what did they have to lose? If they did nothing, the Warmaster would die. At least this way Horus had a chance of life. But there were those amongst the Legion that argued at what price would they buy that life? By bringing him to a house of false gods? The Emperor told them that civilisation would only achieve perfection when the last stone of the last church fell upon the last priest. This went against everything that they had fought for these last two centuries.

The Warmaster Corrupted
It was on Davin, within the blasphemous fane, that things began to turn, where the momentum that led to the corruption of Horus and his Luna Wolves came to a head, planting the seeds of the monstrous betrayal that was to come. Horus fell and then he rose, healed by the arcane powers of the Davinite witches after he agreed within the Warp to betray his father the Emperor in return for mastery over the galaxy, as delivered to him through service to Chaos. From the day of Horus' resurrection on, he took the good and open nature of Luna Wolves' warrior lodge and turned it slowly to meet his own ends, using it as the instrument of corruption to determine which of his Astartes would join him in rebellion -- and which would have to be eliminated before the betrayal could be consummated. Within the warrior lodges of the Luna Wolves and the other Legions targetted by Horus to join his cause, dark shadows grew over the hearts of warriors who had once been devoted and loyal to the Imperium's highest ideals.

After Horus' "miraculous" resurrection, the proliferation of these warrior lodges quickly spread amongst the other Legions, especially those that maintained close bonds with the XVI Legion and eventually turned Traitor. Though these warrior lodges had venerated no god or occult principle, the ritual and secret elements of the warrior lodges did not fit with the ruthless rationality of the Imperial Truth and were soon turned to the purposes of those who sought to spread the worship of the entities of the Warp through the Legions. Frowned upon but tolerated, the lodges persisted and flourished even in the years before Horus' corruption. They had survived in part because many of the officers within the Legions saw them as relatively harmless, and in part because they did promote fellowship within and between Legions. It was a fatal misjudgement that would have dire consequences for all Mankind.