Vortex can be cast three times in a game.
Vortex is useful on water maps as it allows the player to teleport every single one of their units (except for Citizens and ships) to an enemy island without need of any Transport Ships. Another use is to prevent an opponent from finishing a Titan Gate or Wonder by sending a scout, such as a Caladria, then teleporting the entire army to destroy the building in question. The purpose is to hit the enemy with everything the player has as it leaves no units behind to defend the player's base. However, it can also be used to beat a safe, hasty retreat if an attack fails. These tactics are useful on any map where large distances must be crossed or where there are natural barriers slowing down units, such as Jotunheim.