Question: Gates of Arkham introduces several new triggers for At Midnight effects, so that the rules coming with Elder Sign core set are not good enough to cover all the cases. Hence,what is the order in which to resolve At Midnight effects?

Answer:
1. Resolve all "At Midnight" effect on all cards currently in play.
a. "At Midnight" Effects on the Ancient One
b. "At Midnight" Effects on Monsters, in order of the Investigators choosing.
c. "At Midnight" Effects on Adventures and Other World cards, in order of the Investigators choosing.
d. "At Midnight" Effects on other cards.
e. "The next time the clock strikes midnight..." effects on the current Mythos card.
2. Draw one new Mythos card.

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NOTE (from Julia): At the actual state of the game, there are no "at Midnight" effects on monsters that I'm aware of; Sam structured the answer so that we have a definitive fix to any possible "At Midnight" conundrum that may ever exist, from here to the end of days (darn, the end of days could be a Midnight effect too?)

Thank you. I'll print it and add it to the game box to have it handy.

You're welcome I'm happy Sam took time to settle this, it's quite an important point in the economy of the game