When the Gods of Chaos scattered the Primarchs through the galaxy, each was touched by the Warp as they traveled through it. Tialoc was "gifted" with extensive body mutations, and left with a rather snake-like appearance: he was covered from head to toe in green scales, his eyes were slitted and unblinking, and his noseless face concealed a pair of venomous fangs and a forked tongue.
Fortunately, the world on which Tialoc landed was home to a cult that worshiped a snake god, and Tialoc was found by members of this cult. Their god had promised that he would send his son to be their champion, and so Tialoc was taken in.
Tialoc measured up to the cult's expectations, and then some, as he led them to victory over the other tribes on their homeworld. When the God-Emperor of Mankind arrived on the nameless jungle world and explained that Tialoc was his son, and offered him command of the Eleventh Legion, Tialoc readily accepted; he had been raised from birth to conquer in the name of his father and god, so he saw the Great Crusade as the natural next step.
Tialoc had been feared and mistrusted by many on his homeworld for his appearance, but he was startled by the cold reception he received from his brothers and their armies. Of the Primarchs, only Horus seemed to fully trust Tialoc, and he made it clear among his Legion that they were to set their prejudice aside. This caused some friction at first, but the two Legions overcame their suspicions and became stalwart allies. This bond of brotherhood was best exemplified by the crusade to take the Anur Sector from the Slann; both Horus and Tialoc were personally present to lead their Legions, and between the Luna Wolves' tactical prowess and the Sky Serpents' skill at jungle fighting, their victory was quick and decisive.
Horus knew of the Desert Fangs' service record against Chaos, and perceived that, if they interfered with his rebellion (what would later be called the Horus Heresy), things would not go as planned. To forestall such an event, he told Tialoc that Rachnus was a traitor, and instructed him to go to the Desert Fangs' homeworld of Voralia, arrest Rachnus, and annihilate the Desert Fangs. Because of his trust in Horus, Tialoc did not think twice before accepting the mission.
Tialoc's fleet was interrupted by the Desert Fangs; Alpharius (or Omegon) had warned Rachnus that Voralia was in danger. As the Sky Serpents were already in orbit, the Desert Fangs chose to fire a few shots and flee, hoping to draw the Sky Serpents after them; Tialoc did not fall for the ruse, and waited until Voralia was leveled before giving chase. This delay was not wasted by the Desert Fangs. When the Sky Serpents' fleet arrived at the remnants of the Isstvan system, they were ambushed and annihilated, save for Tialoc; it was only a last-minute decision by Rachnus himself that spared him.
Tialoc had already been deeply shaken over the bloodbaths he had witnessed on Voralia and around Isstvan, and learning that he had been manipulated by Horus all but destroyed him. He had been duped into murdering an innocent world, bringing about the ruin of two Legions, and, as he had caused the Desert Fangs to be diverted from the initial attack on Isstvan, possibly destroyed the only chance of averting galaxy-wide civil war. He spent much of the journey to Terra in meditation, and decided to redeem himself and his Legion by participating in the Desert Fangs' final assault on the Vengeful Spirit, Horus's flagship. He took a different path from the Desert Fangs strike force; he did not find Horus, though he fought his way through thousands of Traitor Marines before the psychic backlash of Horus's death caused the Vengeful Spirit to disintegrate.
As the Vengeful Spirit prepared to flee to the Eye of Terror, Tialoc managed to escape in a traitor dropship, but the daemon-possessed vehicle would not obey his commands, and it fled to the Warp before Tialoc could contact Imperial forces. As he was the last of the Sky Serpents, his Legion was, by any reasonable measure, dead and irrecoverable. Their records were placed into a deep archive, and lost in the tumult following the Horus Heresy.
When Tialoc emerged from the Warp, he found himself in a transformed Imperium, where the Emperor was worshiped as a god and the Space Marine Legions had been divided into Chapters. Tialoc felt some measure of responsibility for the Emperor's death, as he had failed to join him on the Vengeful Spirit, and so he resigned himself to a lonely one-Primarch crusade against Chaos.
However, the Sky Serpents' tithed gene-seed was still in storage (though its origins had been lost to the ages), and was used in a later Founding. The Librarians of the resulting Chapter, the Rainbow Warriors, felt instinctively that their Primarch was alive, and Tialoc felt that he was not alone, either. The two happened to cross paths later, and while Tialoc did not feel worthy to lead a Legion again, he taught the Rainbow Warriors all he knew, and they parted ways with a sense of closure: the Rainbow Warriors had solved the mystery of their ancestry, and Tialoc knew that, in some sense, his Legion still lived on.
Tialoc has heard persistent rumors of a Legion of Space Marines who are all nulls, led by a mighty warrior with enough anti-psychic power to challenge a Daemon Prince. Most dismiss those rumors as sightings of Grey Knights distorted by many retellings, but Tialoc knows better: they are signs that Rachnus Rageous is still active. As the forces of Chaos grow bolder, Tialoc's resolve to join his brother and repay his debt to Rachnus and the God-Emperor grows, too, and his present course is taking him inexorably towards the Eye of Terror.
Rules for Horus Heresy
Legiones Astartes (Sky Serpents)
- Units with this special rule may always attempt to regroup regardless of casualties
- Eyes of the Serpent: Members of the Sky Serpents possess the Night Vision special rule.
- Jungle Fighters: Members of the Sky Serpents possess the Move Through Cover and Stealth (Forests) special rules.
- Viper's Venom: Any Sky Serpents model with the Character type may take the Viper's Venom upgrade for five points. This upgrade bestows the Poisoned (4+) rule on all of the character's close-combat weapons that are not power weapons, force weapons, power fists, lightning claws, or thunder hammers - Any weapon with a power field, in essence.
- Sky Serpents; Tialoc was Primarch of this Legion.
- Rainbow Warriors; an old GW Space Marine Chapter, which has lately been envisioned by fans as Aztec Marines.
- Lost Primarch Quest; a quest in which Tialoc was a major NPC.
- Tlaloc (sic) Quest; a quest which attempted to explore Tialoc's early life. It didn't get far.