Rancid Remains has 100 health and spawns in Black Forest.
It takes extra damage from blunt and fire; it resists pierce and frost; poison does nothing to it at all. Slash lands at full damage, so that is what to bring.
Its only attack is mace, 30 poison and 20 blunt.
Some of what it throws is elemental, so poison resistance is worth having.
Of everything craftable, 
Thunderblood Sledge, 
Frostfire Sledge and 
Staff of Embers put the most damage through what it resists.
It always drops 
Bone Fragments.
Rancid Remains is a larger undead skeleton that spawns one at a time in the Burial Chambers of the Black Forest. It has 100 health, is weak to blunt and fire, resists pierce and frost, and is immune to poison. Its mace swings sideways in a 2.2 meter arc every 3 seconds for 20 blunt and 30 poison, so most of the damage arrives after the hit lands. It drops 3 Bone Fragments on every kill and a 
Rancid Remains Trophy 10% of the time.
Same rules as skeletons, blunt and fire, but respect the poison: at Black Forest health pools a single tag hurts. Sidestep the slow swing and answer with a mace of your own.
With devcommands enabled, this spawns Rancid Remains in front of you. The third number is the level: 3 spawns a 2-star.
Creature ID: Skeleton_Poison · all console commands and IDs