Usurper

If you would cause the King to be defeated,
reveal your role and exchange it for the king's
role card. Your life total becomes 40 and the
former King's life total becomes 10 and all
commander damage is removed off him. The
former King's life total can not change until his
next turn. The spells or abilities on the stack can
not cause you to win the game.
Your role must be kept a secret.
When you are defeated, reveal your role.