A Monster Surge is an event caused by the above symbol appearing on a green Mythos card. The Coming Storm Prelude card will also trigger a Monster Surge before gameplay starts.
When a Monster Surge is resolved, the players check if any Gate has the same Omen as the current one in play. If there is at least one, the players place a number of Monsters equal to the number on the reference card on each one of those gates. Monsters cannot be spawned from the Monster cup if the Monster cup is empty. Set aside Monsters cannot be spawned if all set aside Monsters of the named type are on the game board. If no gate corresponds to the Omen, the players spawn 1 Gate.