According to the Players section of the Silicon Knights Blood Omen FAQ: 'An ancient seer once prophesied the march of an enormous army', one that would bring: 'rape and torture, death and destruction on cataclysmic levels', in its unstoppable wake. This seer knew the army - which would be named 'the Legion of the Nemesis' by Nosgoth's people - would have its origins in the north. Unfortunately for everyone, the dire prophesy proved to be accurate. The Legions of the Nemesis featured in Blood Omen. They were the brutal army of the tyrant king - the Nemesis.
The Nemesis had originally been known as William the Just - a kind, gentle and fair monarch. However, as Kain was growing up in Nosgoth, William was conspiring with Moebius the Time Streamer, and through Moebius' "insidious designs", William became a butcher, intent on dominating Nosgoth entirely. He built an army to achieve his ambitions, leading them in "beheading, disembowelling, [and] impaling peasants and warriors alike in their very own villages". By the time Kain reached the age of thirty, and became a vampire, the Legions of the Nemesis were unbeatable.
In Blood Omen, Kain approached the Oracle of Nosgoth to learn how to defeat the Sarafan Malek. The Oracle - actually Moebius - directed Kain towards Vorador, but also spoke of the plight of King Ottmar of Willendorf. Ottmar had the only army in the land that could try to halt the advancing Legions of the Nemesis, but Ottmar was grieving for his daughter, whose soul had been stolen by Elzevir. Later in his travels, Kain reclaimed the soul of the princess of Willendorf, and in return Ottmar roused his Army of the Last Hope.
In the Battle of the Last Stand, the Legions of the Nemesis fought against the Army of the Last Hope. Ottmar was slain and his army defeated. With this victory, there was nothing to stop the Nemesis conquering all of Nosgoth. However, history was changed by Kain, preventing this. By going back in time and killing the young King William, The Legions of the Nemesis were never truly created. William died before he became the tyrant Nosgoth had feared.
The seer who foretold the coming of the Legions of the Nemesis may have been the the Seer from Blood Omen 2. There is no evidence to be sure one way or the other, but it's plausible.