Lendarn is a hostile mage in the Firewine Ruins, and killing him is 1/2 of the side quest The Firewine Ruins. He first casts Improved Invisibility on himself, then switches to offensive spells, starting with Lightning Bolt.

When killed he drops:

(in BG:EE you get Ghoul Touch instead of Chill Touch)
  • a Quarterstaff
  • 159 gp

Lendarn can be "pulled up" the staircase into Jenkal's basement, as can the Ogre Mage. The other enemies in the area (ogrillons and kobolds) will not pursue up the stairs.