I am setting up to play EH with the Strange Remnants expansion. During set-up you are told to draw a Prelude card. The Prelude card I drew was the Dunwich Horror. It states that I draw the "Spawn of Yog-Sothoth" Mystery card if Yog-Sothoth is the AO (which he is). So far, so good.

Then it states that if Y-S is not the AO, spawn the Dunwich Horror Epic Monster on Arkham. But he IS the AO, so I don't spawn the DH Epic Monster on Arkham? The "Spawn of Y-S" Mystery card states that when this card enters play, spawn the DH Epic Monster on Arkham. So I do spawn the DH Epic Monster on Arkham?

If I don't spawn the DH Epic Monster, is this Mystery immediately solved?

As you can tell, I am confused here. Can you guys help me out here? Thanks.

I haven't done this prelude card specifically but the ancient one specific preludes are binary. if you are playing the ancient one you do the first part, but if not do the second part. It sounds like the DH card is covering the bases of the scenario and making sure whether or not you are playing against YS you have to put the DH epic monster into play.

Let me know if that doesn't make sense.

With YS though you get the associated mystery to solve otherwise an epic monster blocking Arkham that doesn't solve a mystery when killed.

I believe the reason the DH is not placed during the set up is due to the fact that you could draw that mystery card. And logically you would fight the monster again, so they leave him out of the set up.

If any other old one is used then as a prelude the dunwich horror is loose in arkham.

If Yog-Sothoth is the Ancient One, then the mystery tells you to spawn the Dunwich Horror.
If Yog-Sothoth is not the Ancient One, then the prelude tells you to spawn the Dunwich Horror.

And that's it.

It works like this:
If Yog-Sothoth is the Ancient One you resolve the first option and ignore what's written in the "If Yog-Sothoth is not the Ancient One" paragraph as it doesn't apply to your game. That paragraph might as well be nonexistent.

The game can be in exactly one of two possible states:
1. Yog-Sothoth is the ancient one.
2. Yog-Sothoth is NOT the ancient one.
And the prelude has exactly two options that correspond to these two situations.

Those options are completely independent from each other, unrelated to each other, don't interact in any way, and are completely exclusive - you are using one or the other, never both.

And you absolutely, completely, don't care about the option that doesn't apply to your game.

Edit: Readability.