In Blood Omen, when Kain returned Azimuth's Eye to the Pillars, Ariel told him of the advancing Legions of the Nemesis, an army from the North who were "crushing all in their path". She echoed the Oracle of Nosgoth's earlier revelations - that King Ottmar and his Willendorf army was the only force that could perhaps stop their progress.
The tyrant known as the Nemesis had once been called King William the Just, and he was a fair monarch, "a caring and gentle benefactor of the land". However, with Moebius' manipulation and encouragement¹, William increased his army in might and number. As his well-trained and ruthless army grew, William ceased to be content with his own northern kingdom. He sought to conquer all of Nosgoth. Kain knew of the bloody and brutal methods William's Legions employed, seeing the effects firsthand when he travelled through Stahlberg. The people had been massacred, impaled and left to rot in the streets. William had become a torturer, and a vicious executioner of anyone who stood in his way.¹
Kain rescued the soul of the Princess of Willendorf, and initiated a confrontation between Ottmar's Army of the Last Hope and the Nemesis' troops. Ottmar died during the Battle of the Last Stand, and the Nemesis' domination of Nosgoth became inevitable. Kain used the Time Streaming device he had found at Avernus Cathedral to escape from the battle before he was overwhelmed by the victorious Legions.
The Time Streaming device carried Kain fifty years into Nosgoth's past. In this time, William was still young, a beloved but weak boy king. Kain saw his chance to save the land from the Nemesis. Against the earlier council of Vorador (not to meddle in human affairs), he entered William's castle, intending to kill him. Kain witnessed that Moebius was acting as an advisor of sorts for the youthful king; he appeared to have William's complete trust, and the Time Streamer had provided the king with weapons, warning him of the vampire assassin that would come.
Kain found and fought William, but both of them were armed with the Soul Reaver. As the two blades met in time and space, a paradox was caused. History was changed, and William died before his natural time. William's Reaver was broken in two, and Kain drank his blood. Kain then left for the present again using a second Time Streaming device. To his surprise, he found Nosgoth even less welcoming than he remembered. His hasty actions had ignited a genocidal war against the vampires. Moebius, who had planned this from the start, had led William's grief-stricken subjects to hunt them. Vorador was executed, and Kain was left as the last vampire in Nosgoth.
In Soul Reaver 2, Raziel saw the tomb of the martyred William in the Sarafan Stronghold. It was in the stronghold's main hall, and there was a fantastic stained glass mural behind it. The mural depicted Kain in battle with the saintly boy king. The broken Soul Reaver was laid on top of William's sarcophagus. As Raziel approached it - carrying the wraith-blade - he felt the temporal distortion the Reavers caused, being in such close proximity to each other. His soul energy was leeched to repair the sword. Later, Raziel spared Kain's life in that chapel, changing history again, and unintentionally impaling William's effigy with the Reaver. Kain commented during that encounter that he had destroyed a tyrant by killing William the Just, only to create one far worse: Moebius, who profited from it all.