The son of Mephisto, Lord of Hatred. Brother to Lilith, which goes by many names. In ages past, several hundreds of years before the events of Diablo I when Sanctuary was still a relatively young world, Lucion ruled the Triune in the guise of the Primus. As the leader of the Triune, Lucion fought a secretive and subtle war against the Cathedral of Light that would decide which of the two cults would eventually dominate the human race. Lucion's plans were eventually thwarted by Uldyssian and his Nephalem followers. Lucion stars in the first of the The Sin War Trilogy by Richard A. Knaak : Birthright.
Some time after the creation of Sanctuary by the renegade angel, Inarius, the world was discovered by the Burning Hells. Realizing that a full scale invasion would inevitably draw attention of the High Heavens (which would very likely result in a heavy struggle that would devastate Sanctuary, rendering it useless to either side), the Lords of the Burning Hells opted for a more subtle approach. They devised a cult that would at first seem to be a harmless and peaceful, but gradually luring its followers deeper into the darkness. The cult, it would turn out, was a dark, unholy sect devoted to the three Prime Evils, at the head of which stood the Primus, the pontifex maximus. However, the Primus was nothing but a façade, a disguise used by the demon Lucion.
The Triune soon grew to become one of the dominating cults of Sanctuary, being second only to the Cathedral of Light, the counter-sect led by Inarius in his guise of the Veiled Prophet. The two sects continued to vie for control over the human race but without any open warfare, as it would draw unwanted attention. Instead, Lucion and Inarius plotted against each other through subtle scheming, always anticipating their opponent's moves.
Their subtle plans were disturbed by the sudden reappearance of Lilith, a renegade demoness who had her own goals and plans. In the guise of the fugitive noblewoman Lylia, Lilith sought to awaken the special powers of the humans (who were in fact the offspring of pairs of renegade angels and demons), harbouring them for her own goals. Her unwitting champion was to be Uldyssian, a simple farmer from the lands near Seram. Noticing Uldyssian due to his immense powers (after it was used to the detriment of several traveling missionaries) and quickly seeing his potential, Lucion dispatched his trusted lieutenant, the high-priest Malic to capture the farmer and his companions. However, unaware of Lilith's presence, the raiding party was overwhelmed and utterly destroyed, sending Malic back fleeing to Lucion. Realizing his sister could have returned, Lucion 'blessed' his failing lieutenant with a demon limb, allowing him to remove Lilith's guise.
After a second and much more powerful raiding party consisting of many undead Morlu attacked Uldyssian and his followers near the town of Partha, but failed again (this time leaving Malic dead by the hands of Lilith), Uldyssian decided to leave the town, after seeing the onslaught. Lilith followed him, still in the guise of Lylia, but soon after their reunion Lucion's spell took effect, revealing Lilith's true form to be a horrified Uldyssian. Lilith easily defeated the farmer, leaving him severely wounded. Still, Uldyssian recovered and reunited with his companions, setting out for the jungles of Toraja.
It was here that Lucion personally intervened. Disguised as a white stallion, the demon tried to lure Uldyssian with him but the latter was timely warned by his brother, Mendeln. Once again taking the form of the Primus, Lucion sought one more time to persuade the farmer to join him. After both attempts failed and Uldyssian and his gathering followers started attacking the demon, Lucion changed tactics. With relative ease the demon incinerated many of Uldyssian's followers and caused the latter's close friend Achillios to die by one of his own enchanted arrows. After seeing his comrades and followers die, Uldyssian's true powers awoke as he burst out in rage. Being unable to maintain his position, Lucion finally revealed his true form: a horrid, reptilian demon with hoofed legs and three tails, covered in thick scales. But even as a mighty demon, Lucion turned out to be overpowered by the sheer force of Uldyssian's abilities. As a last desperate act the demon even offered his obedience and subservience to the farmer, but all for naught. Instead of killing the demon, Uldyssian simply imagined the demon to be nothing, and so the great Lucion faded away and disappeared.
It is unknown what exactly became of Lucion. Though the books are clear on the fact that he was not 'killed' but simply 'was no more', it is never explained what that meant. It could very well be that Lucion was simply banished back into the Burning Hells where he would have to answer to his father Mephisto or maybe Uldyssian's awesome display of force even banished Lucion to a plane beyond the reach of both Heaven and Hell.