Born of a Saxon woman named Erce and the Saxon warlord Aelle, Derfel and his mother were captured in a raid by Britons and enslaved. Their British captors were later raided by the British King of Siluria, Gundleus, and his Druid, Tanaburs. Derfel was to be one of twelve sacrifices to the gods as a tribute from the victorious raiders. Derfel survived being thrown into the "Death Pit" and is henceforth considered by Merlin to be favoured by the gods. Having survived the failed sacrifice, he is also entitled to take the life of the Druid Tanaburs.
Derfel was then adopted by Merlin into his community on the Tor at Ynys Wydryn (Glastonbury) where he became friends with Nimue, otherwise known as Vivien, another child believed to be touched by the gods and Merlin's apprentice. He was present at Caer Cadarn for the birth of Mordred, and was sent to Morgan when Christian priests were unable to safely orchestrate the child's birth. He then travelled to the meeting at Glevum where the guardians of Mordred were selected. Following the death of Uther the Tor was attacked by Gundleus, and Derfel was forced to escape with Nimue. While escaping he took the sword of his dead sword master and later named it "Hywelbane" after his dead master. On the way to Caer Cadarn, Derfel killed his first man when the convoy was attacked by sentries and Derfel goes to attack them. Derfel then meets Arthur for the first time in front of Caer Cadarn.
Derfel joined Owain's troops, and participated in an illegal raid on Kernow for which Owain was killed by Arthur. Derfel then joined Arthur's spearmen and after several years became one of Arthur's warlords and acquired the surname of Cadarn, which means "powerful". Derfel took his troops to Brittany in place of Arthur in order to help fortify King Ban at Ynys Trebes (Mont St. Michel) against the Frankish people. Derfel met Lancelot and Galahad, and found Merlin in Ynys Trebes. When the city was taken by the Franks, Galahad returned to Dumnonia with Derfel and Merlin on Merlin’s boat.
Upon returning to Dumnonia, Derfel rescued Nimue from the Isle of the Dead, which is where the insane were exiled. After rescuing Nimue, he fought in the desperate battle at Lugg Vale. After Arthur's army won the battle, Princess Ceinwyn of Powys, who had previously been betrothed to Arthur until his elopement with Guinevere, chooses Derfel and they become, in essence, husband and wife. Under an oath to Merlin he and a small contingent of his warriors aided Merlin in a successful quest for the most powerful of the thirteen mythical Treasures of Britain, a Cauldron (based on the Quest for the Holy Grail). Afterwards, Derfel continued his Warlord duties to Arthur, fighting in numerous battles against the Saxons, and against Lancelot's rebellion, including playing a crucial role at the battle of Myndd Baddon. His final action was at the Battle of Camlann in which Arthur was taken away in an enchanted boat after being, apparently, mortally wounded.
In his old age Derfel retires to a monastery in Powys where he befriends the queen of Powys, Igraine, wife of King Brochvael of Powys who also happens to be the grand nephew of Derfel's life partner, Ceinwyn. The queen asks him to write his story for her, and Derfel writing his memoirs forms the frame of the series. As the novels progress it becomes apparent that the Saxons are approaching Derfel's monastery. As an ex-warrior, Derfel is ordered by the bishop Sansum to defend the monastery. Although it is not written, it is surmisable this is how Derfel dies.
Derfel bears a strong resemblance to Sir Bedivere. Like Derfel, Bedivere is one of Arthur's closest advisors and friends, is one-handed (in Welsh legends) and in many versions of the legends, casts Excalibur into the sea after the Battle of Camlann.