King Arthur's family in the context of "Knights of the Round Table"

Mordred or Modred (/ˈmɔːrdrɛd/ or /ˈmoʊdrɛd/; Welsh: Medraut or Medrawt) is a major figure in the legend of King Arthur. The earliest known mention of a possibly historical Medraut is in the Welsh chronicle Annales Cambriae, wherein he and Arthur are ambiguously associated with the Battle of Camlann in a brief entry for the year 537. Medraut's figure seemed to have been regarded positively in the early Welsh tradition and may have been related to that of Arthur's son. As Modredus, Mordred was depicted as Arthur's traitorous nephew and a legitimate son of King Lot in the pseudo-historical work Historia Regum Britanniae, which then served as the basis for the subsequent evolution of the legend from the 12th century. Later variants most often characterised Mordred as Arthur's villainous bastard son, born of an incestuous relationship with his half-sister, the queen of Lothian or Orkney named either Anna, Orcades, or Morgause. The accounts presented in the Historia and most other versions include Mordred's death at Camlann, typically in a final duel, during which he manages to mortally wound his own slayer, Arthur. Mordred is usually a brother or half-brother to Gawain; however, his other family relations, as well as his relationships with Arthur's wife Guinevere, vary greatly.

In a popular telling, originating from the French chivalric romances of the 13th century and made prominent today through its inclusion in Le Morte d'Arthur, Mordred is a power-hungry son of Arthur from the incest with Morgause, prophesied by Merlin and destined to bring Britain to ruin. He survives Arthur's attempt to get rid of him soon after his birth and, years later, joins his half-brothers Gawain, Agravain, Gaheris and Gareth in Arthur's fellowship of the Round Table as a young and immoral knight. Eventually, Mordred learns of his true parentage and becomes the main actor in Arthur's downfall. He helps Agravain to expose the illicit love affair between Guinevere and Lancelot and then takes advantage of the resulting civil war to make himself the high king of Britain, ultimately leading to both his own and Arthur's deaths in their battle. Today, he remains an iconic character in many modern adaptations of Arthurian legend, in which he usually appears as a villain and the archenemy of Arthur.

Agravain or Agravaine (/ˈæɡrəveɪn/ AG-rə-vain) is a Knight of the Round Table in Arthurian legend, whose first known appearance is in the works of Chrétien de Troyes. He is the second eldest son of King Lot of Orkney with one of King Arthur's sisters known as Anna or Morgause, thus nephew of King Arthur, and brother to Sir Gawain, Gaheris, and Gareth, as well as half-brother to Mordred.

Agravain secretly makes attempts on the life of his hated brother Gaheris starting in the Vulgate Cycle, participates in the slayings of Lamorak and Palamedes in the Post-Vulgate Cycle, and murders Dinadan in the Prose Tristan. In the French prose cycle tradition included in Thomas Malory's Le Morte d'Arthur, together with Mordred, he then plays a leading role by exposing his aunt Guinevere's affair with Lancelot, which leads to his death at Lancelot's hand.

Morgause (/ˈmɔːrɡoʊz, -ɡɑːz/ MOR-gohz, -⁠gahz) is a popular name of a legendary queen and member of King Arthur's family in the Matter of Britain literature, where she is always a queen and usually Arthur's sibling. However, her name varies between texts and traditions, including Anna, Gwyar, or simply as the Queen of Orkney, as does the issue of her children, other than commonly Gawain. In most cases, she is the wife or widow of Lot, ruling over a northern realm such as Orkney, Lothian, Scotland, or Norway. She often has sisters, notably Morgan, with whom she is being sometimes conflated into a single character by modern authors.

In medieval chronicles and Arthurian romances based on or inspired by Geoffrey of Monmouth's Historia Regum Britanniae, as well as in the Welsh tradition, she is typically depicted as the daughter of Igraine, either the daughter or step-daughter of Uther Pendragon, and the full sister or half-sister of Arthur. Occasionally, she may be Uther's sister and Arthur's aunt. Her other children often, but not always, include Mordred.

Excalibur is the mythical sword of King Arthur that may possess magical powers or be associated with the rightful sovereignty of Britain. Its first reliably datable appearance is found in Geoffrey of Monmouth's Historia Regum Britanniae. Excalibur as the "sword in the stone" functioning as the proof of Arthur's lineage is an iconic motif featured throughout most works dealing with Arthur's youth since its introduction in Robert de Boron's Merlin. The sword given to the young Arthur by the Lady of the Lake in the tradition that began soon afterwards with the Post-Vulgate Cycle is not the same weapon, but in Le Morte d'Arthur both of them share the name of Excalibur. Several similar swords and other weapons also appear within Arthurian texts, as well as in other legends.