The Sith artifact dates back pre-Sith Wars around the time of the first Jedi Schism; there is no knowledge on how the Sith weapon was crafted, but what is known about it is that even the Sith themselves fear the weapon, and that's why it was hidden from the Galaxy. The Necromonus can create the Necro-Plague or formally the Necromonus Plague. The stronger the Force user the stronger the Necromonus and better the range of terror that it can wreak, but only a true Dark Lord can harness the power of the Necromonus.