What if someone attacks with Master of Cruelties and another creature?

Question

My mate constantly plays Master of Cruelties which can only attack alone,

Master of Cruelties isn't forced to attack, but if it does, it must do so alone. If you control another creature with an ability that says it must attack if able, that creature must attack and Master of Cruelties won't be able to.

If Master of Cruelties attacks alone, another creature entering the battlefield attacking will have no effect on it. Master of Cruelties continues to be an attacking creature. The other attacking creature will assign combat damage normally, regardless of whether Master of Cruelties's last ability triggers.'

He always makes the mistake and attacks with multiple creatures with Master of Cruelties. What would the ruling be on this? I am unsure but I would guess maybe it just taps out?

Answer 1

Attacking with Master of Cruelties and other creatures is an illegal action, so they should just undo it and declare attackers again. Specifically, from rule 508.1c:

The active player checks each creature he or she controls to see whether it's affected by any restrictions (effects that say a creature can't attack, or that it can't attack unless some condition is met). If any restrictions are being disobeyed, the declaration of attackers is illegal.

Illegal actions don't have consequences in game, they are just reversed. Rule 717.1 says:

If a player realizes that he or she can’t legally take an action after starting to do so, the entire action is reversed and any payments already made are canceled.