I have Hissing Miasma on the battlefield and my opponent has Kaalia of the Vast and attacks me and puts a Serra Angel tapped and attacking me off the trigger. Does my opponent lose 1 life for the one creature that was declared an attacker (Kaalia), or does my opponent lose 2 life because tapped and attacking qualifies for "attacking" for Hissing Miasma?
508.3a An ability that reads “Whenever [a creature] attacks, . . .” triggers if that creature is declared as an attacker. Similarly, “Whenever [a creature] attacks [a player or planeswalker], . . .” triggers if that creature is declared as an attacker attacking that player or planeswalker. Such abilities won’t trigger if a creature is put onto the battlefield attacking.
Hissing Miasma triggers only once.