Niemain is a mage who is found upstairs in the Sorcerous Sundries shop.
When the player goes upstairs in the Sorcerous Sundries, they will find Niemain and his companions practicing a spell, and he will hail you on sight and castigate you for having "all but ruined" his "extremely difficult spell." If you don't apologize and leave immediately, Niemain and his three companion mages (Oulam, Wheber Ott and William Garst) will attack, unless Xzar and/or Montaron are in the party. In that case, he will mention that Xzar has sent him reports about you and that you should investigate the Iron Throne headquarters in the city. If Xzar is alive, you can ask him about Niemain and his group, and he will reveal that Niemain is a Zhentarim and Xzar's superior.
As Niemain wears The Jester's Folly, a cursed ring of clumsiness, he has -50% Dexterity and a 75% chance of spell failure during the battle, making him the least-threatening of the group — he will attempt to cast his repertoire of ten spells, then engage the party in melee combat with his knife. Niemain is heavily implied to be the "ill-humored mage in Zhentil Keep" that is mentioned in the ring's description.
Killing the four mages will give you a total of 5400 XP.
Once combat has begun, Nieman and the other 3 mages will pursue the party downstairs and out into the street if necessary.