The Nemesis is one of the villains from Blood Omen: Legacy of Kain.
William the Just was a kind and benevolent king, beloved by his people. During this time, Moebius the Time-Streamer became his advisor, encouraging him to raise his army. His army grew massively in numbers, also becoming more well-trained. William slowly changed from a gentle king into a power-hungry tyrant, starting a coup to take over Nosgoth completely. His army became known as the Legions of Nemesis, leaving a path of destruction behind them.
The vampire Kain, after killing Azimuth the Planer, learned of the Nemesis's intentions, and following the advice of the ghost of Ariel and the Oracle of Nosgoth (which in fact was Moebius), went to Willendorf to meet the King Ottmar and get his army to fight the Legions of Nemesis. After rescuing the soul of Ottmar's princess that was under the possession of the dollmaker Elzevir, Ottmar rallied his army to fight the advancing troops from the north, in what became known as the Battle of the Last Stand. Kain fights as well, but the Legions of Nemesis outnumbers Ottmar's army, as Ottmar himself dies. Kain flees by using his Time Streaming Device, ending fifty years in the past. He takes the opportunity and decides to kill William before he becomes the Nemesis. Kain sees Moebius talking with William, warning him that a vampire would come to kill him and giving him a sword to fight.
Kain finds William and fights him, noticing that William is armed with the Soul Reaver, the same sword Kain was using. As the two swords meet, a paradox is created, changing history. William's Soul Reaver breaks in two, as Kain kills him and drinks his blood. William dies a martyr, never becoming the Nemesis. When Kain returns to his time, he realizes that he were playing for a fool this entire time, removing a threat just to create one far worse: Moebius had manipulated him all along to kill William, which fueled his subjects to start a genocidal war against the vampires, leaving Kain as the last vampire remaining.