RWBY: Grimm Eclipse was originally a fan made game developed by Jordan Scott. It soon caught the attention of RWBY's creator Monty Oum, who loved it enough to get Jordan hired at Rooster Teeth to create an official version of the game alongside a team at the newly created video game development arm of the company.
The events of Grimm Eclipse take place in between Volumes 2 and 3 of the show.