Fallen Warrior has 750 health and spawns in the Deep North.
It takes extra damage from spirit; it resists blunt, slash, pierce and fire; poison does nothing to it at all.
Its hardest hit is fw staff fireball at 120 blunt and 120 fire, out of 9 attacks.
Some of what it throws is elemental, so fire, lightning, poison and spirit resistance is worth having.
Of everything craftable, 
Lightning Strike, 
Frostfire Crossbow and 
Frostfire Sledge put the most damage through what it resists.
Fallen Warrior is an Undead creature raised at a North Memorial Place by placing three 
Memorial Coal on the memorial's altar, up to three per site. It has 750 health and resists blunt, slash, pierce and fire, is immune to poison, and is weak to spirit, so a spirit weapon does the work that physical damage barely starts. It is the only creature that drops 
Frostfire Essence and 
Thunderblood Essence, each on an independent 50 percent roll per kill, and it carries no trophy. It walks at 1.6 meters per second and runs at 7.
Undead, resistant to blunt, slash, pierce and fire, immune to poison, and weak to spirit. That is an unusual profile: your physical weapons all land at reduced value and the answer is the eitr staffs you probably stopped carrying after the Mistlands. Bring one. Those two essences gate all twelve Frostfire weapons and all twelve Thunderblood weapons, from the daggers up to the greatswords, so every Memorial Place you clear is a weapon tier.
With devcommands enabled, this spawns Fallen Warrior in front of you. The third number is the level: 3 spawns a 2-star.
Creature ID: FallenWarrior · all console commands and IDs