The Necromancer is located in the UnderKingdom UD Temple.
Within his lair, there are Banshees that cannot be killed and will continue to curse. There are also several adds that appear and can cast Fireball which will break dark. The necromancer attacks at random and hits hard.
The best strategy is to have a wizard who has Lightning Lance which can be cast on all of the hexes adjacent to the portal. Dark 3x3 must be kept up to avoid being attacked with curses.
The Necromancer is known to drop a powerful Scythe, an upgraded F/I Ring that provides stun resist and HP regen, gems, and a mug.