When Entourage of Trest enters the battlefield, you become the monarch.
Entourage of Trest can block an additional creature each combat as long as you're the monarch.
8/23/2016 Entourage of Trest checks if you’re the monarch only as blockers are declared. If it blocks two attacking creatures, another player becoming the monarch won’t change or undo that block.
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.