Manshoon was an archmage and the founder of the Zhentarim until he was ostensibly slain by Fzoul Chembryl, his ally, and Lord Orgauth in 1370 DR.
Fitting the archetype of the sly-tongued, sinister and decadent master spellcaster, Manshoon ruthlessly crushed revolts within his own ranks of the Black Network, easily slaughtered his rivals and weeded the weak out of the ranks of the young and ambitious by sending upstarts on nigh-impossible suicide missions.
He has, allegedly, somehow returned to claim leadership of the Zhentarim on the Sword Coast and move toward full domination of the region.