Basically the owl was deity, so in battles and wars (which were super ceremonial for them), a Priest would often show up dressed like an owl and oversee the battle. Well, I guess in their last stand (after Montezuma was killed) they show up to battle the Spanish and they have a dude in an owl costume that does this big dance etc. Cortes men killed him instantly.

That was basically the last stand. It freaked out the Aztecs and they basically gave up. Apparently in wars against other groups, the owl guy was off limits. And the Spanish killed him within literally 5 minutes.