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", which followed the outbreak of the great Plague of Justinian in Constantinople by a few years. 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 4 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.

Answer this question "What is he known for during his reign?" by extracting the answer from the text above.
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.