Why does this work? It’s because “leaves the battlefield” triggers have to “look back in time,” so to speak, at the game state immediately before an event in order to check whether they should trigger. This is contrary to all the other kinds of triggers, which check the game state immediately after an event to check for triggers.
This isn’t only true for creatures. Anything that looks for permanents leaving the battlefield works the same way. So if you have a single event that destroys Dark Prophecy at the same time as four creatures, like Planar Cleansing does, the game looks back to just before the event and sees that there was a Dark Prophecy with a trigger that cared about the creatures that just died at the same time as it was destroyed. You’ll get 4 triggers, and will draw 4 cards and lose 4 life.
Today’s Rules Tip written by Josh Stansfield