From Lexicanum

The Beta was a battleship used by the Alpha Legion Adeptus Astartes in the years of the Great Crusade.

It served as the primary flagship of Alpharius himself and made a journey with the Imperial expedition for a meeting with the Cabal. Once the meeting was concluded, the Beta turned its weapons against the Imperial forces and was supported by its action by its sister ship Alpha. ¹

Source

• ¹:Legion