Marchesa, the Black Rose has the following ability:
Whenever a creature you control with a +1/+1 counter on it dies, return that card to the battlefield under your control at the beginning of the next end step.
I had a number of creatures on the battlefield, including Marchesa herself, with one or more +1/+1 counters on them when Supreme Verdict was played, destroying each creature on the board.
When Marchesa and several other creatures would be killed simultaneously,
Does Marchesa's ability trigger for herself?
Does Marchesa's ability trigger for the other creatures under my control with +1/+1 counters?
In attempting to resolve the issue, we tried to look things up, leading to this forum post, which cites Rule 603.6d as proving that Marchesa's ability should trigger both for herself and for each other creature with +1/+1 counters.
Unfortunately, none of us could actually understand Rule 603.6d, aside from this example:
Two creatures are on the battlefield along with an artifact that has the ability Whenever a creature dies, you gain 1 life. Someone plays a spell that destroys all artifacts, creatures, and enchantments. The artifacts ability triggers twice, even though the artifact goes to its owners graveyard at the same time as the creatures.
The example certainly seems analogous to me, but without understanding the actual rule, rather than just the example, we weren't too sure.
A complete answer has to not only answer the question, but back up that answer with citation and explanation of the official rules on the matter.