As I was playing the other day a beautiful mythos card that said: All monster in street area are return to the cup. This in its self is clear, but when it happened Dunwich Horror was on the board (on a street) and 3 Servant of Glaaki were also in the board and in a street at the time.
What should I do in this case? Take them from the board? They are monsters and they are on the street.

Some AO have a special ability like Ithaqua that says: “In addition, all Weather cards are discarded without their special effects taking place.”
This means:

1. I should discard the and nothing takes place?
2. I should discard the first part of the mythos where it says what the weather effect is but still apply the rest (clue token, open gate and monster movement)?
3. Discard that mythos and keep drawing until a non weather comes up and resolve it as normal?

Atlach-Nacha has the ability that all gate openings are gate bursts. And as we know each time there is a gate burst all flying monster move. The questions is the following regarding fliers and they movement:

1. With every mythos all fliers move regardless of where the gate appears?
2. It is a gate burst only if it breaks an elder sign (and fliers will move only in this case)?
3. It is a gate burst every time a gate opens regardless if the location has an elder sign (so the fliers will only not move when the location has already a gate)?

Thanks in advance for the answers
Best Regards,
Night

Spawn monsters are unaffected by effects that return monsters to the cup. Ithaqua causes you to apply all of the effects of a Weather Mythos except for the Environment effect. Cthugha, on the other hand, makes you discard and re-draw a Weather Mythos. The bit about gate bursts and fliers is infamously unclear, but I would play it that all fliers move whenever a gate opens or a monster surge occurs.

Yeah, flyers do move every bloody round!

Walk said:

Thx to the quick response. As for the spawn monsters not been affected by return to the cup effects, is this on the rules or was later clarify?

It was later clarified, so that effects like "Feds Raid Arkham" wouldn't automatically eliminate the Dunwich Horror.