Drawing an extra mythos card at midnight is a common way of adding difficulty. We felt that was a bit much, so we created heralds that have a chance of adding a mythos card (we aimed for about a 50% chance in "normal" play of adding a mythos card).

There are 16 heralds. Some have you discard items to try and avoid mythos cards, some have using specific types of items increase the chances. Some make you want to defeat monsters, some make you want to avoid monsters. One makes you want to succeed at certain adventure cards, one makes you want to avoid certain adventure cards.

You can modify your play to try and avoid extra mythos cards, or just ignore it and let the extra mythos cards come. Since they top out at one extra mythos card per midnight, the impact is limited.

You can switch the effect to adding a doom token instead of drawing an extra mythos card if you want to up the difficulty.

https://boardgamegeek.com/filepage/143262/elder-sign-heralds