Complex sentence: He is unsympathetic to humans, despite the fact that he used to be one, until he meets his soulmate Rashel Jordan, who happens to be the most feared vampire hunter in "Night World" history.
Split into simpler sentences: He is unsympathetic to humans despite the fact that he used to be one. This is, until he meets his soulmate Rashel Jordan who happens to be the most feared vampire hunter in "Night World" history.