So, getting straight to the point! When Ilharg attacks and Marchesa BR is out on the field, does the creature that is tapped and attacking with Ilharg get the dethrone trigger? I was just told it does not and I cannot find a source that supports his claim. I assumed that all creatures that were attacking get the dethrone trigger as long as it satisfies its stipulation of attacking the player with the most life.
Nope. Dethrone specifies that it occurs when it ATTACKS, which any creatures that enter the battlefield already attacking didn't do. It never went from a state of not attacking, to a state of attacking - it was always attacking. Put another way, it was never declared as an attacker.
Hopefully that makes sense. Anyway, it's a definite no.