The Hydra Galaxy is a galaxy that moves through the universe.
At a currently unknown point in Earth's history, a race of Super-Ancient Beings discovered the Hydra Galaxy, which was hurtling through the universe aimlessly on the crest of a gravitational wave. They were subsequently able to learn how to change the direction of the Hydra galaxy as they saw fit.
The Super-Ancients knew that even after they were gone from the Earth (by extinction, or otherwise departing the planet), there may yet be intelligent life, and decided to set the Hydra galaxy on a collision course with Earth as the third of five trials, which would allow them to determine if a new sentient species had arisen and were worthy of existing.
Since 10,000 BC, the Hydra Galaxy has made preliminary passes of the Earth on three occassions. Each time, the Four Legendary Kingdoms held the Great Games of the Hydra in order to practise the ritual for the time when the Hydra Galaxy would be on a direct course for Earth as part of the Ancients' third trial. Upon the success of each of these Games, a signal would be sent to the Hydra Galaxy, which in response would send out a signal containing information from the Ancients known as the Mysteries, which would provide whoever recieved them with knowledge regarding the final two trials.
The Hydra Galaxy is in the shape of a tetra-gammadion / swastika, and is 400 times larger than the Milky Way Galaxy. Because it moves on the crest of a gravitational wave originating from from the centre of the universe, the Hydra Galaxy is moving at 10 times the speed of sound. What keeps it from expanding devastatingly as it moves is the Hydra Galaxy's negative density, which draws in its matter as fast as its being flung.
- The shape of the Hydra Galaxy inspired the myths regarding the multi-headed Hydra beast because of the shapes of its arms.