The Saxon leader in the Elizabethan play The Birth of Merlin. He takes the role of Hengist from the chronicles.
His sister, Artesia, married King Aurelius Ambrosius. Ostorius used his position as the king’s brother-in-law to move hordes of his troops to Britain. He then betrayed and murdered Aurelius. He was slain by Uther Pendragon in battle.
Source
The Birth of Merlin, or the Childe Hath Found His Father | Attributed to William Shakespeare and William Rowley, 1662