From Anglo Saxon England, by Sir Frank Stenton, ISBN 0-19-282237-3, page 4: One of the few primary sources of early Anglo-Saxon England is the British monk, Gildas, who wrote shortly before 547. He was reluctant to introduce personal names into his writing. The few British kings he did introduce by name were those whose flagrant sins invited commination. A king who sinned in moderation had no interest for Gildas. His longest and most intense invective is devoted to the greatest of contemporary rulers, Maelgwn of Gwynedd, lord of Anglesey.

From Wikipedia:
Maelgwn son of Cadwallon (Latin: Maglocunus; Welsh: Maelgwn ap Cadwallon; died c. 547), better known as Maelgwn Gwynedd or Maelgwn the Tall (Maelgwn Hir), was king of Gwynedd during the early 6th century. Surviving records suggest he held a pre-eminent position among the Brythonic kings in Wales and their allies in the "Old North" along the Scottish coast. Maelgwn was a generous supporter of Christianity, funding the foundation of churches throughout Wales and even far beyond the bounds of his own kingdom.
Nonetheless, his principal legacy today is the scathing account of his behavior recorded in De excidio et conquestu Britanniae by Saint Gildas, who considered Maelgwn a usurper and reprobate. Maelgwn's original name was presumably the Brythonic *Maglocunen. Maglocunen meant something like "Princely Hound", with the positive military connotation of a dangerous warhound.
After the collapse of Roman authority in Britain, north Wales was invaded and colonized by Gaelic tribes from Ireland. The kingdom of Gwynedd began with the reconquest of the coast by northern Britons under the command of Maelgwn's great-grandfather Cunedda Wledig. Generations later, Maelgwn's father Cadwallon Long-Hand completed the process by destroying the last Irish settlements on Anglesey. Maelgwn was the first king to enjoy the fruits of his family's conquest and he is considered the founder of the medieval kingdom's royal family. He is thus most commonly referenced by appending the name of the kingdom to his own: Maelgwn Gwynedd. By tradition, his llys (English: royal court, literally hall) was located at Deganwy, in the Creuddyn peninsula of Rhos. Tradition also holds that he died at nearby Llanrhos, and was buried there. Other traditions say that he was buried at Ynys Seiriol (English: Island of St. Seiriol, Puffin Island), off easternmost Anglesey. There are no historical records to confirm or deny these traditions.
Historical records of this early era are scant. Maelgwn appears in the royal genealogies of the Harleian genealogies, Jesus College MS. 20, and Hengwrt MS. 202. His death in a "great mortality" of 547 is noted in the Annales Cambriae. Tradition holds that he died of the 'Yellow Plague' of Rhos, but this is based on one of the Triads that was written much later. The record says only that it was a "great mortality".
Maelgwn was a generous contributor to the cause of Christianity throughout Wales. He made donations to support Saint Brynach in Dyfed, Saint Cadoc in Gwynllwg, Saint Cybi in Anglesey, Saint Padarn in Ceredigion, and Saint Tydecho in Powys. He is also associated with the foundation of Bangor, but hard evidence of this is lacking. In his 1723 Mona Antiqua Restaurata, Henry Rowlands asserts that Bangor was raised to an episcopal see by Maelgwn in 550, but he provides no source for the assertion.
The only contemporary information about the person is provided by Gildas, who includes Maelgwn among the five British kings who he condemns in allegorical terms in his De Excidio et Conquestu Britanniae. He says Maelgwn held a regional pre-eminence among the other four kings, going on to say that he overthrew his paternal uncle (Latin: avunculus) to gain the throne; that he had taken up life as a monk but then returned to the secular world; that he had been married and divorced, then remarried to the widow of his nephew after being responsible for his nephew's death; and that he was tall.
The evidence suggests that Maelgwn held a pre-eminent position over the regions ruled by the descendants of Cunedda, perhaps in the sense of a regional high king. There is nothing to suggest that Maelgwn held sway over any larger area. Gildas says as much in his condemnation, saying he held a pre-eminence over the other four kings similarly condemned, and also describing him as the "dragon of the island", where the Isle of Anglesey is the ancient stronghold of the kings of Gwynedd.
The fact that Maelgwn's donations to religious foundations are not restricted to the Kingdom of Gwynedd but are spread throughout northern and southern Wales in the regions where the descendants of Cunedda held sway implies that Maelgwn had a responsibility to those regions beyond the responsibilities of a king to his own kingdom. Taliesin also implies it, in his Marwnad Rhun (English: Elegy of Rhun) that laments the death of Maelgwn's son Rhun, where he says that Rhun's death is "the fall of the court and girdle of Cunedda"
In his De Excidio et Conquestu Britanniae (English: On the Ruin and Conquest of Britain), written c. 540, Gildas makes an allegorical condemnation of five British kings by likening them to the beasts of the Christian Apocalypse as expressed in the biblical Book of Revelation, 13-2: the lion, leopard, bear, and dragon, with the dragon supreme among them. He says that Maelgwn is the "dragon of the island", and goes on with a litany of moral accusations, in the process describing him almost as a regional high king over the other kings (the power-giving dragon of the Apocalypse). The Isle of Anglesey was the base of power of the kings of Gwynedd, so describing Maelgwn as the "dragon of the island" is appropriate.
Gildas restricts his attention to the kings of Gwynedd (Maelgwn), Dyfed (Vortiporius), Penllyn (probable, as its king Cuneglasus/Cynlas appears in royal genealogies associated with the region), Damnonia/Alt Clud (Constantine), and the unknown region associated with Caninus. The Welsh kingdoms are all associated with the conquest of the Gaels by Cunedda, while Alt Clud had a long and ongoing relationship with Gwynedd and its kings. In the course of his condemnations, Gildas makes passing reference to the other beasts mentioned in the Apocalypse, such as the eagle, serpent, calf, and wolf. The reason for Gildas' disaffection for these individuals is unknown. He was selective in his choice of kings, as he had no comments concerning the kings of the other British kingdoms that were thriving at the time, such as Rheged, Gododdin, Elmet, Pengwern/Powys, or the kingdoms of modern-day southern England. That he chose only the kings associated with one king's pre-eminence (Maelgwn, the "dragon") suggests a reason other than his claim of moral outrage over personal depravity. Neither outrage nor a doctrinal dispute would seem to justify beginning the condemnation of the five kings with a personal attack against the mother of one of the kings, calling her an "unclean lioness". In the Historia Brittonum, Nennius says that "the great king Mailcun reigned among the Britons, i.e., in Gwynedd". He adds that Maelgwn's ancestor Cunedda arrived in Gwynedd 146 years before Maelgwn's reign, coming from Manaw Gododdin, and expelled the Scots [i.e., the Gaels] with great slaughter.
Maelgwn is not mentioned in the Welsh Triads, but the pestilence that killed him appears as one of the 'Three Dreadful Pestilences of the Isle of Britain'. It is described as the Yellow Plague of Rhos, originating from the carcasses of the dead.
There is an incidental mention of Maelgwn in the song To Maenwyn found in the Red Book of Hergest and attributed to Llywarch Hen. The steward (Welsh: maer) Maenwyn is encouraged to resist a command to surrender his post and show his fidelity to Maelgwn. In the Book of Llandaff, compiled c. 1125, Maelgwn Gwynedd is claimed to be one of the benefactors of the Diocese of Llandaff in its early years. One of the specific places mentioned is at Louhai (Tintern parva, some 6 miles north of Chepstow), where Maelgwn is claimed as a secular witness to its donation.