Hexen has 800 health and spawns in the Meadows, the swamps, the mountains, Black Forest, the plains, the Ashlands, the Deep North, the ocean and the Mistlands.
It takes extra damage from fire; it resists frost; poison and spirit do nothing to it at all. Blunt, slash and pierce land at full damage, so that is what to bring.
Its hardest hit is magicblast at 120 lightning and 30 blunt, out of 6 attacks.
Some of what it throws is elemental, so lightning and spirit resistance is worth having.
Of everything craftable, 
Thunderblood Crossbow, 
Lightning Strike and 
Staff of Embers put the most damage through what it resists.
It always drops 
Nornathread.
With devcommands enabled, this spawns Hexen in front of you. The third number is the level: 3 spawns a 2-star.
Creature ID: JotunWitch · all console commands and IDs