Legend tells that Pandora's box released all the evils of mankind when it was opened. But in reality the box actually is a weapon of ultimate strength that decimated societies and crushed civilizations until the Council of 98, a secret order, took the relic and its secret and hid it from world. They hid it so good in fact, that over time the knowledge was lost even to them and so the world was safe from the box's destruction. At least until a team of archaeologists find the box on the bottom of the ocean, recover it and place it in a museum in New York City.

The player takes on the role of Charles Deckard, a professional art thief. He has been commissioned by a mysterious man named LeFey to get into the museum and steal the box but instead of stealing it once he is in front of it, he makes the biggest mistake in his life and opens it unleashing horros upon the world which were believed to be only myths. Now Deckard is hunted by a mysterious entity named "The Black Order" and simultaneously has to undo the disaster he created.

Legendary is a straight forward first-person-shooter. During the course of the game Deckcard finds everything from an axe to molotov cocktails and an advanced assault rifle to fight against the ancient horror and special units of the Black Order. When he opened the box he also gained the ability to see and use the life energy called Animus Vitae. Dropped by the mythical monsters after their death, it not only heals Deckard but can also be used to unleash an energy wave called Animus Pulse which blasts nearby enemies to the ground.

When the player has enough of all the world-saving, he can start up the multiplayer mode for up to 8 players. The game pits the rebels against the Black Ops but also puts in the mythical, AI-controlled monsters from the single player campaign to grant the players access to Animus Vitae and generally spice things up.