When Crown-Hunter Hireling enters the battlefield, you become the monarch.
Crown-Hunter Hireling can't attack unless defending player is the monarch.
8/23/2016 Crown-Hunter Hireling can attack only the monarch or a planeswalker controlled by the monarch. This check is done only as attackers are declared. After that point, Crown-Hunter Hireling won’t be removed from combat if that player ceases to be the monarch.
8/23/2016 The game starts with no monarch. Once an effect makes one player the monarch, the game will have exactly one monarch from that point forward.
8/23/2016 Abilities that trigger whenever you “become the monarch” trigger only if you aren’t already the monarch. For example, if you are already the monarch as Custodi Lich enters the battlefield, its last ability won’t trigger.
8/23/2016 If the triggered ability that causes the monarch to draw a card goes on the stack, and a different player becomes the monarch before that ability resolves, the first player will still draw the card.