I just got done discussing this elsewhere, but I thought it might come in handy when working with the Archfiends. You may or may not know it, but hey, others stumbling here may like to know:
As you continue down your path of Archfiend dueling, one thought may run across your mind...
Ooh, here's an idea. Since I have Pandemonium on the field, when my Terrorking is destroyed, I can use Pandemonium's effect to search my deck for a Desrook, and THEN use that Desrook to revive my Terrorking!!!
Unfortunately you cannot do things this way. Let's take a moment to look at a couple things...
Pandemonium Ruling:
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.
Desrook Ruling:
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.
The key thing to look at here is that Desrook's effect is OPTIONAL. That means that the last thing to happen on the event or chain link is that Terrorking has to be send to the graveyard for the potential Desrook revival. However, it is possible to get too monsters out of the deal. Let's see what happens when we start building the chain.
Link 1 is Pandemonium's effect to search for a monster, and link 2 is Desrook's revival effect (since we need to have Terrorking going to the graveyard as the last thing to happen). So we resolve in reverse...
Resolution 1: Desrook is sent to the graveyard and Terrorking is revived
Resolution 2: A monster of 3 stars or less is searched for in the deck and brought to the hand.
If we tried to do this in reverse order, this is what would happen:
Resolution 1: A monster of 3 stars or less (assume Desrook) is searched for and brought to the hand.
Resolution 2: Desrook is sent to the graveyard to...hold up, the last thing we had happen was a monster was searched for and brought to the hand. So we can no longer use Desrook's effect, because the timing is invalid.
Hopefully this clarifies a few things for you. The key to remmeber is that Desrook's effect is optional, which causes a few restrictions on when it can be activated. This same concept also applies to Archfiend of Gilfer in certain scenarios.
Hopefully this has clarified a point for you and feel free to ask any more questions if needs be.