From the Official WH40K page:
Astartes is a Warhammer 40,000 animation that follows a Space Marine strike force as it executes a bloody operation.
Assaulting an enemy vessel, the Adeptus Astartes face hordes of foes, cunning witchcraft, and terrifying unknown technology as they seek to bring rebellious enemies to justice.
This is the Youtube original upload without the awful edits from the official release.