Charred Warlock has 600 health and spawns in the Ashlands.
It takes extra damage from spirit; it resists pierce, fire and frost; poison does nothing to it at all. Blunt and slash land at full damage, so that is what to bring.
Its hardest hit is fire at 100 pierce, out of 2 attacks.
Of everything craftable, 
Lightning Strike, 
Thunderblood Sledge and 
Thunderblood Greatsword put the most damage through what it resists.
It always drops 
Charred Bone.
Charred Warlock is a Demon faction caster found in Charred Fortresses, up to three to a fortress. It has 600 health and adds frost resistance to the usual Charred profile of reduced pierce, near-useless fire, poison immunity and a spirit weakness. Its staff throws a green fireball for 100 pierce every 8 seconds that bursts into a 3.5 meter fire vortex lasting 7 seconds, and every 10 seconds it raises a 
Summoned Twitcher within 5 meters of itself, to a cap of ten shared across every loaded warlock. It drops 1 to 3 Charred Bone and the 
Warlock Trophy at 5%.
Kill the warlock first. Every summon is a fresh twitcher, and the green bolts pierce for 100 through whatever you hid behind. They resist fire and frost both; spirit and blunt do the talking.
With devcommands enabled, this spawns Charred Warlock in front of you. The third number is the level: 3 spawns a 2-star.
Creature ID: Charred_Mage · all console commands and IDs