Bagdemagus was the brother of both Donadix and Tarsan, all nephews of King Uriens of Gore. He was the eldest of the trio and inherited his uncle’s kingdom upon when he died. The brothers were also, apparently, King Arthur’s cousins, presumably on the maternal side in both cases.

Early in Arthur’s reign, Prince Bagdemagus was a major opponent of the British King. First, he tried to replenish his depopulated country by preventing all visitors from leaving. He then joined his uncle in open rebellion, only capitulating when Saxon raids forced them to make peace. He moved on to support King Galehaut of Sorelais in his wars against Arthur. But eventually Bagdemagus became a firm supporter of the Arthurian Court and companion of his cousin, Sir Yvain, and also of Gawain. He had hoped to join the Round Table but, when a seat eventually became available, it was given to Sir Tor. Somewhat put out, he stormed out of Camelot and rode off into the forest. Stopping at a wayside cross, his squire observed an inscribed prediction that he would not return to court until he was an equal of the Round Table Knights. Bagdemagus agreed that he would certainly not go back until he had the respect of the Order which had rejected him.

The young prince rode off in search of adventure. He found the holy herb that pointed the way to the Holy Grail. He discovered Merlin’s rocky prison and spoke to the old wizard but was unable to lift the stone which barred his escape. In all respects, he proved himself a good and honourable knight and, when he finally returned to Camelot, he was indeed made Knight of the Round Table.

As a monarch, Bagdemagus was not so triumphant in subsequent exploits. Sir Kay defeated him in combat and hung him upside down from a tree; and he slept with King Pellinore’s wife and was beaten near to death by her cuckolded husband. Perhaps the discovery of the grail herb encouraged Sir Bagdemagus to enter into the search for the Holy Grail. Unfortunately, this too was not altogether successful. He attempted to take up the Adventurous Shield and was struck down by a warrior angel who had reserved it for Sir Galahad. He was later killed during the quest, after an unfortunate misunderstanding. Bagdemagus had come across Sir Mordred in the middle of raping a young maiden. He wounded the wicked knight in combat, but was later pursued by the latter’s brother, Gawain. Ignorant of both the circumstances and Bagdemagus’ identity, Gawain killed him.

King Bagdemagus had several daughters and a son, Sir Meleagant, with whom he was very close. He once hired a knight named Sauseise to unseat him early in a tournament in order to save him from further possible harm. Being a good and just King, Bagdemagus tried to protect Queen Guinevere when she was kidnapped by his son. Yet, poor Sir Lancelot was still very nervous about telling the man that he had killed Meleagant during the lady’s rescue.

Literary writers seem to have been in some confusion as to the location of Bagdemagus' kingdom. He is usually described as a King of Gore: a land inherited from his supposed uncle, King Uriens. However, his son, Sir Meleagant, is clearly placed in the Summer Country - meaning Somerset - and Bagdemagus is said to have held court at Bath. It is true that Gore itself is difficult to pin down, but it is usually accepted at the historic Urien's kingdom of Rheged covering modern Cumberland and Westmorland. The name of Bagdemagus's character in the "Culhwch and Olwen" is given merely as Baeddan. Perhaps this was a nickname taken because he lived at Caer-Baeddan (Bath). At any rate, it would seem that his real name was Baeddan de Magus. Magis was the Roman fort at Burrow Walls in Rheged. However, Magus could be a corruption of Magnis, the Roman town of Kenchester in Herefordshire. If Bagdemagus/Baeddan came from Herefordshire, he may have become embroiled in the war against the Saxon Mercians fought largely by the King of Pengwern in adjoining Shropshire. It is known that a branch of this Royal family fled to Eastern Dumnonia and founded the sub-kingdom of Glastening, which roughly corresponds to Somerset. Perhaps Bagdemagus went with them.