Achillas was one of the guardians of the Egyptian king Ptolemy XIII Theos Philopator, and commander of the king's troops, when Pompey fled to Egypt in September 48 BC. He was called by Julius Caesar a man of extraordinary daring, and it was he and Lucius Septimius who killed Pompey at the suggestion of the eunuch Pothinus and Theodotus of Chios.