Question: King Arthur is a legendary British leader who, according to medieval histories and romances, led the defence of Britain against Saxon invaders in the late 5th and early 6th centuries AD, the sparse historical background of Arthur is gleaned from various sources, including the writings of Gildas, a 6th-century British monk, best known for his scathing religious polemic titled what?

Context paragraphs:
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.

Aurelius Conanus: Aurelius Conanus or Aurelius Caninus was a Brittonic king in 6th-century sub-Roman Britain.  The only certain historical record of him is in the writings of his contemporary Gildas, who excoriates him as a tyrant.  However, he may be identified with one of the several similarly named figures active in Britain during this period.  In the 12th century Geoffrey of Monmouth adapted Gildas' account for his chronicle "Historia Regum Britanniae", and thereafter Aurelius Conanus was remembered as a legendary King of Britain.

Groans of the Britons: The Groans of the Britons (Latin: "gemitus Britannorum" ) is the name of the final appeal made by the Britons to the Roman military for assistance against Pict and Scot raiders.  The appeal is first referenced in Gildas' 6th-century "De Excidio et Conquestu Britanniae"; Gildas' account was later repeated in Bede's "Historia ecclesiastica gentis Anglorum".  According to Gildas, the message was addressed to the general Flavius Aetius and requested his aid in defending formerly Roman Britain from the Picts and Scots.  The collapsing Western Roman Empire had few military resources to spare during its decline, and the record is ambiguous on what the response to the appeal was, if any.  According to Gildas and various later medieval sources, the failure of the Roman armies to secure Britain led the Britons to invite Anglo-Saxon mercenaries to the island, precipitating the Anglo-Saxon settlement of Britain.

Eucheria: Eucheria was a Roman poet who was active sometime in the late 5th to early 6th centuries AD.  She is notable for being, along with Sulpicia, a rare example of a Roman female satirists.  One poem of hers survives, an epigram that ridicules a lower-class suitor.  The poem is composed of 16 elegiac couplets, which play on the theme of courtship between classes with the juxtaposition of objects of high and low quality, such as a "noble Punic tunic and an awful shaggy rag."  Vocabulary in the poem suggests that she may have been from Gaul, particularly the court in Aquitania.  Attempts to identify her with historically attested women named Eucheria active in Late Antique Gaul have been inconclusive.

St Twrog's Church, Bodwrog: St Twrog's Church is a small rural church at Bodwrog in Anglesey, North Wales.  Built in the late 15th century in a medieval style, some alterations have been made but much of the original structure still remains.  It has two 15th-century doorways (one later converted into a window) and some 15th-century windows.  The bull's head decoration used on the church denotes a connection with the Bulkeleys of Beaumaris, a prominent north Wales family over several centuries.  Set in a remote part of the countryside in the middle of Anglesey, it is dedicated to St Twrog, who was active in the late 5th and early 6th centuries.  The church's tithes were paid for at least two hundred years to Jesus College, Oxford, which has historically strong links with Wales, and the college at one point built a house for the priest who served St Twrog's and a neighbouring parish.

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.

Budic II of Brittany: Budic II (Medieval Latin: "Budicius" ; Welsh: "Budig" or "Buddig "; c. 460 – c. 550 ), formerly known as Budick, was a king of Cornouaille in Brittany in the late 5th and early 6th centuries.  He was the father of Hoel Mawr and is probably to be identified with the Emyr Llydaw ("Emperor of Brittany") and King Nentres who appear in Arthurian legend.  Upon his death, his kingdom was usurped by Macliau, king of the neighboring Veneti.

King Arthur's messianic return: King Arthur's messianic return is an aspect of the legend of King Arthur, the mythical 6th-century British king.  Few historical records of Arthur remain, and there are doubts that he ever existed, but he achieved a mythological stature that gave rise to a growing literature about his life and deeds.  One recurrent aspect of Arthurian literature was the notion that he would one day return in the role of a messiah to save his people.

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.

King Arthur: King Arthur is a legendary British leader who, according to medieval histories and romances, led the defence of Britain against Saxon invaders in the late 5th and early 6th centuries AD.  The details of Arthur's story are mainly composed of folklore and literary invention, and his historical existence is debated and disputed by modern historians.  The sparse historical background of Arthur is gleaned from various sources, including the "Annales Cambriae", the "Historia Brittonum", and the writings of Gildas.  Arthur's name also occurs in early poetic sources such as "Y Gododdin".

Supporting paragraphs: King Arthur Gildas
Answer: De Excidio et Conquestu Britanniae