Missing the Timing: Because “Desrook Archfiend”’s effect is optional, the “Terrorking Archfiend” being sent to the Graveyard has to be the very last thing that happened in order for you to activate “Desrook Archfiend”’s effect. You cannot activate “Desrook Archfiend”’s effect in the middle of a chain, such as if “Pot of Greed” was activated and “Ring of Destruction” was chained to destroy “Terrorking Archfiend” and send him to the Graveyard in the middle of the chain.
You can activate “Desrook Archfiend”’s effect when a face-down “Terrorking Archfiend” is destroyed and sent to the Graveyard by a card effect.
“Desrook Archfiend” is a Multi-Trigger Effect and can be activated during your opponent’s turn, and even during the Damage Step.
When you control your opponent’s “Terrorking Archfiend” with “Change of Heart”, and it is destroyed, you can activate “Desrook Archfiend” from your hand to Special Summon the “Terrorking Archfiend” back to your side of the field.
While “Pandemonium” is active, and you have “Desrook Archfiend” in your hand, and your “Terrorking Archfiend” is destroyed while on the field, “Pandemonium” is Step 1 of a chain , but you can use “Desrook Archfiend” as Step 2 of the chain. Then you would get to Special Summon the “Terrorking Archfiend” and select a Level 3 or lower Archfiend from your Deck.
Netrep Rulings:
If multiple “Terrorking Archfiends” are destroyed at the same time, only one “Desrook Archfiend” may be discarded to Special Summon one “Terrorking Archfiend.”
“Desrook Archfiend”, if retrieved by the effect of “Pandemonium” from a “Terrorking Archfiend” being destroyed and sent to the Graveyard, may not be used to Special Summon “Terrorking Archfiend” from the Graveyard.