Question: Ambrosius Aurelianus as a war leader of the Romano-British who won an important battle in the 5th century, according to a 6th-century British monk best known for his scathing religious polemic which recounts the history of the Britons before and during what? 

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Ambrosius Aurelianus: Ambrosius Aurelianus (Welsh: "Emrys Wledig" ; Anglicised as Ambrose Aurelian and called Aurelius Ambrosius in the "Historia Regum Britanniae" and elsewhere) was a war leader of the Romano-British who won an important battle against the Anglo-Saxons in the 5th century, according to Gildas.  He also appeared independently in the legends of the Britons, beginning with the 9th-century "Historia Brittonum".  Eventually he was transformed into the uncle of King Arthur, the brother of Arthur's father Uther Pendragon, and predeceases them both.

Inverness: Inverness ( ; from the Scottish Gaelic: "Inbhir Nis" ] , meaning "Mouth of the River Ness") is a city in the Scottish Highlands.  It is the administrative centre for the Highland council area, and is regarded as the capital of the Highlands.  Inverness lies near two important battle sites: the 11th-century battle of Blàr nam Fèinne against Norway which took place on The Aird and the 18th-century Battle of Culloden which took place on Culloden Moor.  It is the northernmost city in the United Kingdom and lies within the Great Glen (Gleann Mòr) at its north-eastern extremity where the River Ness enters the Moray Firth.  At the latest, a settlement was established by the 6th century with the first royal charter being granted by Dabíd mac Maíl Choluim (King David I) in the 12th century.  The Gaelic king Mac Bethad Mac Findláich (MacBeth) whose 11th-century killing of King Duncan was immortalised in Shakespeare's largely fictionalized play "Macbeth", held a castle within the city where he ruled as Mormaer of Moray and Ross.

Felec of Cornwall: Felec or Felix was an obscure 5th- or 6th-century British saint active in Cornwall.  The church of St Felicitas and St Piala's Church, Phillack near Hayle is dedicated to Saint Felec (as he appears in a 10th-century Vatican codex).  Later generations mistook him for the females Saint Felicity (alias Felicitas) and Saint Piala.  Saint Felix was said to have had the miraculous gift of being able to communicate with lions, cats, and other feline creatures.  There is also a Mount St Phillack in Victoria, Australia not far from Mount St Gwinear.

Riothamus: Riothamus (also spelled Riutimus or Riotimus) was a Romano-British military leader, who was active circa AD 470.  He fought against the Goths in alliance with the declining Roman Empire.  He is called "King of the Britons" by the 6th-century historian Jordanes, but the extent of his realm is unclear.  Riothamus is a Latinization of the Brythonic personal name "*Rigotamos", meaning 'king-most', 'supreme king' or 'highest king'.  Though it is still a matter of debate, several scholars consider his life to have been one of the possible sources for the King Arthur legend.

Battle of Badon: The Battle of Badon (Latin: "Bellum in monte Badonis" or "Mons Badonicus", Welsh: "Cad Mynydd Baddon" , all literally meaning "Battle of Mount Badon" or "Battle of Badon Hill") was a battle thought to have occurred between Celtic Britons and Anglo-Saxons in the late 5th or early 6th century.  It was credited as a major victory for the Britons, stopping the encroachment of the Anglo-Saxon kingdoms for a period.  It is chiefly known today for the supposed involvement of King Arthur, a tradition that first clearly appeared in the 9th-century "Historia Brittonum".  Because of the limited number of sources, there is no certainty about the date, location, or details of the fighting.

Romano-British culture: Romano-British culture is the culture that arose in Britain under the Roman Empire following the Roman conquest in AD 43 and the creation of the province of Britannia.  It arose as a fusion of the imported Roman culture with that of the indigenous Britons, a people of Celtic language and custom.  It survived the 5th century Roman departure from Britain.  Scholars such as Christopher Snyder believe that during the 5th and 6th centuries – approximately from AD 410 when the Roman legions withdrew, to AD 597 when St Augustine of Canterbury arrived – southern Britain preserved an active sub-Roman culture that survived the attacks from the Anglo-Saxons and even used a vernacular Latin when writing.

De Excidio et Conquestu Britanniae: De Excidio et Conquestu Britanniae (Latin for "On the Ruin and Conquest of Britain") is a work by the 6th-century British cleric St Gildas.  It is a sermon in three parts condemning the acts of Gildas' contemporaries, both secular and religious, whom he blames for the dire state of affairs in sub-Roman Britain.  It is one of the most important sources for the history of Britain in the 5th and 6th centuries, as it is the only significant source for the period written by a near contemporary of the people and events described.

Gildas: Gildas (Breton: "Gweltaz", c. 500–570) — also known as Gildas the Wise or Gildas Sapiens — was a 6th-century British monk best known for his scathing religious polemic "De Excidio et Conquestu Britanniae", which recounts the history of the Britons before and during the coming of the Saxons.  He is one of the best-documented figures of the Christian church in the British Isles during the sub-Roman period, and was renowned for his Biblical knowledge and literary style.  In his later life, he emigrated to Brittany where he founded a monastery known as St. Gildas de Rhuys.

Lucius of Britain: Lucius (Welsh: "Lles ap Coel") is a legendary 2nd-century King of the Britons and saint traditionally credited with introducing Christianity into Britain.  Lucius is first mentioned in a 6th-century version of the "Liber Pontificalis", which says that he sent a letter to Pope Eleutherius asking to be made a Christian.  The story became widespread after it was repeated in the 8th century by Bede, who added the detail that after Eleutherius granted Lucius' request, the Britons followed their king in conversion and maintained the Christian faith until the Diocletianic Persecution of 303.  Later writers expanded the legend, giving accounts of missionary activity under Lucius and attributing to him the foundation of certain churches.

Merlin: Merlin (Welsh: "Myrddin" ) is a legendary figure best known as the wizard featured in Arthurian legend and medieval Welsh poetry.  The standard depiction of the character first appears in Geoffrey of Monmouth's "Historia Regum Britanniae", written c. 1136, and is based on an amalgamation of previous historical and legendary figures.  Geoffrey combined existing stories of Myrddin Wyllt (Merlinus Caledonensis), a North Brythonic prophet and madman with no connection to King Arthur, with tales of the Romano-British war leader Ambrosius Aurelianus to form the composite figure he called Merlin Ambrosius (Welsh: "Myrddin Emrys" ).  He is allegedly buried in the Broceliande forest, near Paimpont in Brittany.

Supporting paragraphs: Ambrosius Aurelianus Gildas
Answer: the coming of the Saxons