Emergencies are a feature added in the upcoming Civilization VI: Rise and Fall expansion pack. Here's the lead designer Anton Strenger on the subject: Most Emergencies get triggered when one player gets a significant lead or advantage in an area. Converting a Holy City to a different religion, or using a nuclear weapon, for example. When triggered, the game determines which other players can join in an Emergency against the target and each player can choose to join or pass. Joining can give permanent benefits, but only if the players are able to complete an Emergency-specific objective against the target in time, otherwise the target gets a benefit instead.