The last ability of Throne Warden checks to see if youâ€™re the monarch as your end step begins. If youâ€™re not, the ability wonâ€™t trigger at all. You wonâ€™t be able to do anything that would make you the monarch during your end step in time to have that ability trigger. The ability will also check to see if youâ€™re the monarch as it tries to resolve. If youâ€™re not the monarch at that time, the ability will have no effect.
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.
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.
Abilities that trigger whenever you â€œbecome the monarchâ€ trigger only if you arenâ€™t already the monarch. For example, if you are already the monarch as Custodi Lich enters the battlefield, its last ability wonâ€™t trigger.