What it does
Dethrone triggers when a creature with it attacks the player with the most life. When that happens, you put a +1/+1 counter on the attacking creature. It only checks at the moment of attack—if you attack the highest-life player, you get the counter; if not, you don't, and life changes after that point don't matter.
The most common misunderstanding: it doesn't matter whether the attack connects or the creature survives—the trigger happens on declaration of attackers, so the counter goes on immediately. Also, if multiple players are tied for highest life, attacking any of them counts. Importantly, attacking a planeswalker controlled by the highest-life player still works, since you're attacking that player's permanent.
In Commander, with high starting life and multiple opponents, Dethrone shines—there's almost always someone above you, so your creatures snowball counters across a long game.








