NK

NK is a penalty for killing players in selected nation maps. They always include towns. Attacking another play in these maps will result in NK points. A single hit from a attack or spell is worth 1 NK point, killing a player is worth 20 NK point.
At 5 NK guards will attack you on sight and at 10 NK you may lose items if you die. You do not gain NK for attacking anyone with at least 1 NK point and lastly, you may lose NK by either waiting for 10 minutes real time (decrease NK by 1) or asking a priest to use anthem (decrease 10 NK per spell cast), however anthem will make you lose experience. Please note that it is possible to get -%!

Further, if your NK is below 10 and a player/guard kills you, you will lose the NK and not lose exp. If you have reached the maximum negative EXP, your currently equipped weapon tactic will decrease upon death but will not be affected by anthem.
Killing the guard gives the same consequence as killing a player (20NK) but hitting a guard does not give NK.

From the help built into the game client.