You can't use "Pandemonium" to add a Fusion Monster Archfiend from the Fusion Deck to your hand, like "Fiend Skull Dragon".
If multiple Archfiends are destroyed at the same time by "Raigeki", you can only add 1 Archfiend from your Deck to your hand with "Pandemonium".
When your Archfiends controlled by your opponent are sent to your Graveyard, you can add an Archfiend from your Deck to your hand with "Pandemonium".