The Serpent Lodge was both a Chaos Cult and a place of worship known as the Temple of the Serpent Lodge or the Delphos that was located on the moon of the world of Davin. The cultists of the Serpent Lodge and their temple was devoted to Chaos Undivided, and was the location where the Warmaster Horus was treated for injuries incurred while quashing a rebellious Chaos force on Davin's moon during the final days of the Great Crusade in the early 31st Millennium.
With Horus on the threshold of death after being wounded by the mutated and Nurgle-corrupted rebel Planetary Governor Eugen Temba who was wielding the potent Chaos weapon known as the Kinebrach Anathame, Horus' senior commanders of the Luna Wolves Legion were desperate to save their Primarch's life. They allowed Horus to be treated by the Davinite Priests who were in truth a Chaos Cult.
Using sorcery, which had been outlawed by the Emperor at the Council of Nikaea, the cultists managed to turn Horus against the Emperor by playing on the seed of jealousy, desire for power and resentment that the Warmaster felt for his father after the Emperor had left the Great Crusade behind to return to Terra.
Magnus the Red, the Primarch of the Thousand Sons Legion, attempted to reveal these manipulations of the Chaos Gods for the lies and falsehoods that they were, but Horus' corruption by the Ruinous Powers could not be stopped as he gave in to his egotism and bitterness.
Once the now-tainted Horus recovered his health, he schemed with the Word Bearers First Chaplain Erebus and the Astartes of the Luna Wolves' Warrior Lodge to overthrow the Emperor of Mankind as the ruler of the Imperium of Man and purge his XVI Legion of its remaining Loyalists, bringing about the great galactic civil war known as the Horus Heresy.
Some six solar 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). Long home to a pre-industrial, barbaric culture, Davin had long been ruled by a remarkable warrior caste, whose savage nobility had won the respect of the Astartes sent to pacify their raging and bloody feuds and bring Davin into the Imperium. 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 Space Marine Legions to match their own internal Legion cultures.
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 soon 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 XVI 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 would 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 other Space Marine Legions 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. Those Astartes within those Legions with lodges who held themselves aloof from those secretive organisations were marked as Imperial Loyalists who would later be betrayed during the Istvaan III Atrocity.
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 that advanced human civilisation.
In the course of that battle against Temba, the potent living metal of the Chaos blade wielded by the plague-infused monstrosity that had once been Temba 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 they felt they had nothing left 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 wondered what price would be paid to buy that life. The Emperor had long taught that human civilisation would only achieve perfection when the last stone of the last church fell upon the last priest. To turn to the savage gods of a primitive, superstitious primitives went against everything that the Astartes of the XVI Legion had fought for over the course of two standard centuries.
Horus Is Corrupted
It was on Davin, within the blasphemous fane known as the Temple of the Serpent Lodge, 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 once good and open nature of the 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 targeted 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 Astartes 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.
Temple of the Serpent Lodge
The Temple of the Serpent Lodge, also known as the Delphos, was an imposing stone building set in the middle of a huge crater on Davin that had a diameter stretching across thousands of metres. The crater was located at the end of a long valley, and the chief way to approach it by land involved travelling along a stone causeway set along the bottom of the valley. The causeway was lined with the crumbling stone statues of ancient Davinite kings.
The actual temple was an octagonal structure, with a bastion and tower built at each corner. A further eight towers surrounded a dome raised in the centre of the structure, and each tower was tipped with a blazing fire. A single large archway led into the dome and the fane itself. The approach to this dome involved climbing a set of massive stone steps that were lined with coal braziers and pillars carved to feature serpents. At the top of the steps was a path that narrowed as it approached the gate that was similarly lined with burning fires and stone pillars. However, between the gated archway and the top of the steps was a wide circular pool. This pool generated three narrow watercourses that rushed towards the steps, and in fact continued down it, with one channel on either side and one set in the middle, splitting the staircase.
The gate itself consisted of two large doors of beaten and carved bronze. They featured a complex design only fully understandable when the gate was closed. It showed a large tree with spreading and fruit-laden branches. Many minor serpents twisted and spiraled around the edges of the design, but two great snakes coiled around the tree and each other, with their heads entwined together above it. When he saw this image, Captain Garviel Loken noted that it was strikingly similar to that worn by Space Marine Apothecaries; the ancient symbol of the caduceus. The paving in front of the gate continued the image of the tree by featuring a carving of three great roots that led to the edge of the wide pool. Apparently, once the gates were closed they could only be opened from within.
There was a secret chamber at the heart of the temple, protected by powerful Chaotic sorcery and screening technology installed by disgruntled Tech-adepts of the Mechanicum in return for information given to them by the Word Bearers Legion. Circular in nature, the chamber had a sacrificial circle and sinkhole built into its centre. It was here that the Lodge members cast the spells that allowed Erebus to communicate with Horus during the wounded Warmaster's stay in the temple.
Davinite belief was that the injured taken inside were left to the care of the spirits of the deceased, and their ultimate destiny would then be revealed -- to die and join the spirits of the dead in the beyond, or to find a second chance at life. They were expected to come forth healed, but if this did not occur after nine days, their remains were brought out and burned, the ashes cast into the pool. The reborn Horus came forth to greet his waiting Astartes upon the ninth day after he was taken into the Delphos.
- Horus Heresy Collected Visions, pp. 54, 127
- False Gods (Novel) by Graham McNeill