What to know
Writhan has 400 health and spawns in the swamps.
It takes extra damage from slash; it resists blunt and fire; poison does nothing to it at all. Pierce lands at full damage, so that is what to bring.
Its only attack is bite, 40 pierce and 30 poison.
Some of what it throws is elemental, so poison resistance is worth having.
Of everything craftable, 
Thunderblood Greatsword, 
Frostfire Greatsword and 
Thunderblood Greataxe put the most damage through what it resists.
It always drops 
Writhan Roots.
Spawn command
With devcommands enabled, this spawns Writhan in front of you. The third number is the level: 3 spawns a 2-star.
Creature ID: Writhan · all console commands and IDs