A calamity is a means of attack used only by prophets. It is a divine power that can really wreak havoc on the enemy. However, if the prophet is within a temple's range, he cannot use a calamity. Many of the calamities can hurt friendly units, so discretion should be used (i.e., don't fire into the middle of a melee battle).
The following is a list of each of the various calamities prophets can cast:
Malaria- Malaria is represented by the prophet casting flies at enemies. Malaria affects all human units and, in a direct hit, causes the unit to be reduced to a single hit point. This is best used on masses, as the enemy can be taken out by a single hit. Costs 100 power.
Firestorm- A firestorm sets fire to a building or ship. The fire takes off HP from the target and is also spread to any building or ship within 1 tile of an afflicted building or ship. It can destroy farming operations very easily! However, it ONLY works on ships and buildings, not walls, units, or temples (as it cannot be cast inside an enemy temple's range). Costs 75 power.
Earthquake- The cheapest and most frequent building-attacker used, earthquakes cause fault lines that rapidly reduce a building's HP. In early epochs, a building can be taken out in a single attack, Unlike the firestorm, this does affect walls. Costs 50 power.
Volcano- When used, a 36 square tile volcano shoots out of the ground and spews rocks in random directions. Volcanoes are excellent defensive calamities, blocking chokepoints and preventing any (land) unit from passing. Costs 60 power.
Plague- A slower yet more destructive effect than malaria, this calamity uses rats instead of flies. Plague-afflicted units have rats running around their feet, chipping HP away until it wears off or the unit dies. It also only affects human units, and can spread to any unit, friend or foe, the carrier comes into contact with. Costs 100 power.
Hurricane- The calamity most often used in naval comnbat, a hurricane, when cast, can be controlled by the player who cast it for a period of time. It sinks ships incredibly quickly and can cut an attacking force to smithereens in no time flat. Costs 100 power.
Meteor Storm- You may not see this often, as it's only available in expansion pack games with space terrain. When available, it causes a bunch of shooting stars that sink spaceships. Costs 75 power.