Rewarded in NPK.
Believe it is just a random yeoman. Other considerations:
Karnifax is paladin/priest, so I don't think it is alignment based, as they probably have +350. Maybe if you PK someone and they are nuetral reputation.
The key line here is: For killing a neutral party, the Powers have granted thee a Mark.
This probably just means killing in NPK.
You settle on a plan.
You ram the crown of your head into Karnifax's face, sending her reeling.
Your fists clench with righteousness as your bash turns to holiness.
Karnifax is struck on the torso with your holy wrath causing massive damage.
Your fists clench with righteousness as your surprise attack turns to holiness.
Karnifax is struck on the torso with your holy wrath causing lethal damage.
Karnifax has been slain!
You smash your head into Karnifax's, killing her instantly.
For killing a neutral party, the Powers have granted thee a Mark.
You have gained the Mark of Manslaughter!
You gain 1250 archon points!
You gain 150 practices!
You feel more evil.