Uldyssian ul-Diomed, son of Diomedes, was one of the re-emergent nephalem of the Edyrem, during the last days of the Sin War. His actions saved Sanctuary from being destroyed and touched Tyrael's heart as well as changing his view of humans being abominations.
"And, of all those most terribly ignorant of Sanctuary's awful destiny, Uldyssian ul-Diomed - Uldyssian, son of Diomedes - could be said to have been the most blind. Blind, though he would be himself at the center of what scholars of the world's secret history would come to call the Sin War."
Uldyssian used to be a simple farmer, tending to his farm on the outskirts of the hamlet Seram. He was accustomed to the hardships of farm life - virtually his entire family was wiped out by a plague, including his parents and several of his siblings. Only Uldyssian and his younger brother Mendeln survived. This left Uldyssian embittered, especially towards the two greatest cults who, despite their promises, could do nothing to save his family. Therefore Uldyssian regarded emissaries of both the Cathedral of Light and the Triune with hostility.
One day when Uldyssian was in Seram, two travelling missionaries, one belonging to the Cathedral of Light and one belonging to Temple of Triune were found brutally murdered. While Uldyssian had nothing to do with it, one of the victims was found with Uldyssian's knife lodged within his ribcage. Uldyssian was charged with murder and put into prison. Somewhere prior to the discovery of the two murder-victims, Uldyssian had met the hauntingly beautiful noblewoman Lylia. Lylia had fled from the city of Kehjan after her family had been stripped of its power and wealth by the Cathedral and the Triune. Though many of her family were thrown in Kehjan's dungeons, Lylia managed to escape. Uldyissian, infatuated by Lylia's appearance, promises to help her.
During his time in prison, his strange powers manifested themselves for the first time as his cell's door spontaneously unlocks by itself.
Later, a party from the Cathedral of Light arrived to oversee the trial. When the Master Inquisitor was about to declare Uldyssian guilty for the two murders, a bolt of lightning conveniently struck the Inquisitor and killed him. As a vicious storm quickly gathered, Uldyssian and Lylia managed to escape amidst the chaos. During their flight, Lylia badly sprained her ankle; however, it mysteriously heals as Uldyssian placed his hands over the bruises. Unconvinced it was his power that healed Lylia, Uldyssian tried to invoke his so-called powers to subdue the storm over Seram. Much to his astonishment, the storm dissipated.
To the townspeople, this proved Uldyssian's guilt at being a murderer and so everyone - even his lifetime friends in the village - rejected him from their lives. Uldyssian and his friends - Serenthia, Achilios and his brother Mendeln - fled town. Lylia convinced Uldyssian to use his powers to bring down both the Cathedral and the Triune, restoring free will to humanity. During their travel to Kehjan, Uldyssian and friends had a hostile encounter with the High Priest Malic, whom they forced to retreat. After the battle, Uldyssian's party ended up in the large town of Partha.
During his stay in Partha Uldyssian revealed to the local townsfolk the potential of raw magic within all of Man. However, he also discovered that Lylia was only a disguise of the demon Lilith and that she claimed that she was responsible not only for what happened in Seram but also all the display of power that Uldyssian had shown so far.
With her mentally mocking and abusing him before leaving him to his fate, she proclaimed just what he would be without her, and left him seemingly powerless. With his confidence in his abilities utterly shattered, the shamed farmer was eventually found by his friends. The party then decided to head out towards the Torajan Jungles where they could lay low. After making camp within the jungle, Uldyssian stumbled upon a large group of Parthans who followed him. Aghast, Uldyssian fled into the wilderness, only to bump into Lucion himself.
During the showdown with Lucion, Uldyssian rediscovered his own powers (with no reasonable explanation). With ease, the farmer dispatched the demon, but his good friend Achilios was slain. Re-convinced that the power of the nephalem was their birthright, Uldyssian and his followers headed out for the city of Toraja, where they intended to raise an army in order to bring down both the Triune and the Cathedral.
Scales of the Serpent
"...The world was forever changed by the second coming of the nephalem, but changed most of all was the first among their kind, Uldyssian ul-Diomed. Wanting nothing more than the simple, worthy life of a farmer, he was now forced to become a catalyst for upheaval. Through him would be revealed some of the truth concerning Sanctuary, as the world was called by those who most vied to control it. Through him did others learn of the eternal war between the angels and demons through the guises of the Cathedral of Light and the Temple of the Triune."
— The Books of Kalan (excerpt)(src)
Shortly after reaching Toraja, the Triune turned against both Uldyssian's followers and the city itself, prompting the nephalem to storm the temple. The temple and its clergy were decimated, but Uldyssian did not manage to destroy the demoness Lilith, who escaped. Still, after leaving Toraja behind, the Edyrem's ranks expanded greatly. After the Edyrem left Toraja they passed by a temple within the jungles. In the jungles, there was an unnameable beast which Mendeln described as a being that has stayed between life and death because of grief and anger, during the process the beast tried to capture Serenthia but was only able to knock her unconscious leaving another trace of guilt in Uldyssian for not being able to protect her. After Serenthia was saved she seemed to have an odd power around her but Uldyssian paid it no mind, but after she was saved Mendeln went missing and the Edyrem had to move on without him.
The Veiled Prophet
"...And with the destruction of the main Temple of the Triune and the vanishing of its master, Uldyssian, son of Diomedes, and his edyrem then spread across the land, purging the last major traces of that sect. The flames of justice and vengeance burned bright together to devour much of what remained of the Cult of the Three."
— The Books of Kalan (excerpt)(src)
In the aftermath of the Triune's fall, Uldyssian and Edyrem set about clearing up its remnants and also setting their sight on the Cathedral of Light. Eventually, he and Inarius met in battle, but it was interrupted with the coming of the Heavenly Host. Angels, having learnt of Sanctuary's existence, were determined to eradicate the "abomination" that was humanity. In turn, the forces of the Burning Hells emerged as well. Thus resulted in what would be the last battle of the Sin War, as angels, demons, and the Edyrem met in a three-way battle. In it, Uldyssian unleashed the whole of his power, driving the forces of Heaven and Hell back to their domains. However, he realized that the power that he and the Edyrem possessed was tearing Sanctuary apart. It was too much, gained too quickly. He feared that he was losing himself, that his nephalem birthright was consuming his humanity.
At that moment, Uldyssian realized the potential for humanity's nephalem powers to do the same as what they had done to him. As such, in a final act, Uldyssian withdrew his nephalem energies back into himself. Choosing humanity over godhood, Uldyssian's energies were released one last time, negating his very being.
"I did call them abominations...and I was wrong! My vote is for them...For I would see what they might become...and marvel in it."
— Tyrael vouches for humanity's continued existence(src)
It is suggested in the Books of Kalan that Uldyssian's release of energy flooded the Worldstone, resetting it, and stripping the Edyrem of their powers. More notably, his act of sacrifice saved humanity in more ways than one, as the Angiris Council met to decide the fate of humankind. Moved by Uldyssian's act of self-sacrifice, Tyrael cast the deciding vote for humanity's continued existence instead of destroying it.
As the Edyrem's memories were removed as a condition of the pact made between Heaven and Hell to allow Sanctuary's continued existence, Uldyssian's actions were mostly forgotten, and it was believed that he had succumbed to a second plague. Mendeln did remember, and he recorded what he could in the Books of Kalan.
Uldyssian became a figure of myth in some cultures/areas of Sanctuary. Since the end of the Sin War, various mystics have witnessed what Deckard Cain theorized as an echo of Uldyssian's sacrifice. He believed that his act echoed on a psychic plane, and could be seen by those in deep meditation. He theorized that it was this echo that inspired Akarat to preach Zakara, and not Yaerius as the ascetic believed.
- His full name appears to have been inspired by Ulysses, a king of Greek Mythology who was said to rule the island of Ithaca somewhere in the Ionian Sea.
- It is possible his name is equally inspired by Odysseus, the original Greek name for the mythical figure who the Romans called Ulysses.