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, split as evenly as possible between the gates.
If no gate corresponds to the Omen, the players spawn 1 Gate.