Chaos players have several ratings. One of them is chaos power rating that is basically a standing among chaos troops.
It's calculated like ELO.
Chaos player can CHALLENGE any other chaos player that is within their power ranking (4,3,2,1) and of higher power rating. Power ranking is determined automatically by certain thresholds.
If the challenger wins, he gains more rating than the defender if he wins. Weaker loser loses less rating than stronger loser.
If the defender declines the challenge, he will basically forfeit without losing gear but the rating loss will be the same.
The winner of the challenge can choose to swap some (or all) of his gear with the loser.
There will be a limit on how often a challenge can be issued per day to prevent spam.
Winner of a challenge cannot land a finishing blow.
Chaos power can be also gained (to a lesser degree) by defeating enemy faction players.
Chaos power is lost slightly by being defeted by enemy players (regardless if a finishing blow is received or not).