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