Quoting the press release:
An unending swarm of enemies awaits those brave enough to take on the game’s Endless Eclipse Mode, where players can prove their worth as they go toe to toe with countless foes in the formidable Eclipse dimension. Offering a unique and challenging respite from the main story, the new mode has players fighting for their life as they rush to complete various missions to overcome the deadly Eclipse. Slashing their way through this realm will gradually present them with stronger opponents and harder missions and pit them to a grueling struggle for survival. Success comes with rewards however, and those who manage to withstand the challenge will find themselves better equipped to tackle the main game mode.