A collection of warlike Asiatic tribes that invaded eastern and central Europe in the fourth and fifth centuries.
Geoffrey of Monmouth says that the Huns, led by Guanius, invaded Britain in the fifth century, during Maximus’s absence, but were driven out by Gratian Municeps. They returned upon Gratian’s death, and were finally defeated by Arthur’s grandfather Constantine.
Source
Historia Regum Britanniae | Geoffrey of Monmouth, c. 1138