To provide some context first: This short was filmed as camera tests with Neill Blomkamp, who was at some point considered to do the HALO Movie. All you see here are basically just camera tests he did during his trial runs.
Halo Legends is a collection of seven short stories relating to the Halo universe. Announced at Comic-Con and financed by 343 Industries, the animation was created by five Japanese production houses: Bones, Casio Entertainment, Production I.G., Studio 4°C, and Toei Animation. Shinji Aramaki, creator and director of Appleseed and Appleseed Ex Machina, serves as the project's creative director.
Halo: Nightfall introduces a pivotal new character, Jameson Locke, a legendary manhunter and agent with the Office of Naval Intelligence (ONI), Earth’s most powerful and secretive military branch. When he and his team are caught in a horrific biological attack, they unravel a plot that draws them to an ancient, hellish artefact, where they will be forced to fight for their survival, question everything, and ultimately choose between their loyalties and their lives. Taking place between the events of Halo 4 and Halo 5: Guardians, Halo: Nightfall is Locke's origin story which will lend insight into the character he becomes in Halo 5.
Depicting an epic 26th-century conflict between humanity and an alien threat known as the Covenant, the series weaves deeply drawn personal stories with action, adventure and a richly imagined vision of the future.