Lucius Tiberius, or Hiberius, is a major foe of King Arthur in numerous earlier versions. He is the leader of the Romans fighting King Arthur, but dies fighting Arthur. In his first appearance, he is a Procurator acting for the Roman Emperor Leo, but in nearly all other appearances, he is Emperor.
Lucius is first mentioned when ambassadors arrive at Caerleon from Rome. They deliver a letter from Lucius Tiberius, Procurator, who states Arthur has not paid the tribute due to Rome and his conquests of Roman lands have also caused anger, meaning he must present himself in Rome for judgement. Arthur refuses this, stating that by the Roman's argument that Britain must pay tribute as Julius Caesar conquered them, Rome should pay tribute to Britain due to previous British Kings conquering Rome.
Arthur crosses over to Gaul with an army, while Lucius calls together an army from the south and east. An embassy including Arthur's nephew Gawain is sent to meet Lucius, but when Lucius' nephew Gaius Quintilianus, insults the Britons Gawain kills him and the embassy flees the camp.
Lucius considers whether to cross to Augustodunum till the Emperor Leo can bring his forces, but decides to confront Arthur. Arthur's forces clash with those of Lucius and Gawain personally fights him, but an assault by the Romans forces Gawain back. Morvid, consul of Gloucester, finally attacks the Romans in the rear, breaking their forces and killing Lucius. Arthur then sends the body of Lucius and other nobles back to Rome, telling them this is the only tribute he will send them. Arthur prepares to cross to Rome and fight the Emperor Leo, but before he can become Emperor he hears his nephew Mordred has usurped the throne of Britain and returns to fight them.
Lucius is referred to as both Tiberius and Hiberius in Geoffrey of Monmouth. Hiberius is a name meaning Spanish. In one of the earliest adaptations from Geoffrey, Wace's Roman de Brut, Lucius is explicitly called Spanish.
Lucius is called Emperor sometimes in Geoffrey's text. However from Wace onwards Leo is excised from the text and Lucius is entirely referred to as Emperor. Leo does appear in the Alliterative Morte Arthure but is a subordinate of Lucius.
Though Lucius fights Arthur towards the end of Arthur's reign in most versions, in Malory's version he is met towards the beginning of Arthur's reign. This may be as Malory wishes to portray the Roman war in a more positive sense, so doesn't want it to be the cause of Arthur's downfall.
Lucius in is La Mort le Roi Artu portrayed more sympathetically. The writer mentions the Romans were very valiant and fought well, and when Lucius is killed by Arthur the author mentions this was tragic as he was a young man and a great Knight, in contrast to Arthur, who in this version is 92 at the time.
Lucius in the Alliterative Morte Arthure is killed by Arthur, while originally it is unknown who killed him.
In the Alliterative Morte Arthure the Gaius killed by Gawain is Lucius' uncle rather then nephew.