When Regal Behemoth enters the battlefield, you become the monarch.
Whenever you tap a land for mana while you're the monarch, add an additional one mana of any color.
8/23/2016 Regal Behemoth’s last ability is a triggered mana ability. It doesn’t use the stack and can’t be responded to.
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.