Challenge is a game mode unlocked at 500 rating.
Challenge is a daily game where you battle against an AI setup. Usually, the player is at a disadvantage in position or have lower morale, but all of them are beatable. If you win the battle, the next day when the next challenge is released, you may claim rubies based on difficulty (how many people failed the challenge), which is a percentage with a certain curve. The reward is 20 rubies max at 95%+ difficulty. Sometimes Challenge has an unusual unit such as Chaosportal, or impossible positions.