In Geoffrey’s chronicle, a Roman war leader in the third century who was sent by the Roman senate to deal with Carausius, another Roman who had betrayed the British king and taken the throne for himself. Allectus succeeded in killing Carausius and assuming the throne.
He then began to slaughter the Britons in return for their support of Carausius. After several years of this bloody reign, Allectus was overthrown and killed by the Briton Asclepiodotus, who succeeded him.
These are real people, but Geoffrey has their situation confused. Allectus served as finance minister to Carausius, who was a Roman admiral rather than a British king. Carausius plotted to overthrow the Roman Emperor Diocletian and invaded Gaul. Allectus murdered him, and was himself slain by the Roman general Constantius Chlorus.
Source
Historia Regum Britanniae | Geoffrey of Monmouth, c. 1138