The original Manshoon was a powerful mage and the founder of the Zhentarim, also known as the Black Network. He was "slain" by Fzoul Chembryl, his ally, and Lord Orgauth in 1370 DR. However, Manshoon had created a dozen or more clones that the murderous duo then found themselves facing. This then led to the "Clone Wars" where the clones battled eachother for supremecy.
It is rumored that only one clone survived.