Question: What Shakespearean play named after a King of the Britons recorded by the medieval scribe Geoffrey of Monmouth included a character  considered a villain, obsessed with power and overthrowing her elderly father?

Context articles: Sisillius I
Eudaf Hen
Welsh mythology
Ódhrán Ua hEolais
Bladud
Cordelia (King Lear)
List of legendary kings of Britain
Goneril
List of legendary rulers of Cornwall
Regan (King Lear)

Sisillius I ("Welsh:" Seisyll) was a legendary king of the Britons as accounted by Geoffrey of Monmouth.
 He was preceded by Gurgustius and succeeded by Jago.
 He was the father of Kimarcus, king of the Britons, and shares his name with one of the sons of Ebraucus, and two later kings of the same name (Sisillius II and Sisillius III).
 Geoffrey has nothing to say of him beyond this.
Eudaf Hen (Eudaf "the Old") or Octavius is a figure of Welsh tradition.
 He is remembered as a King of the Britons and the father of Elen Luyddog and Conan Meriadoc in sources such as the Welsh prose tale "The Dream of Macsen" and Geoffrey of Monmouth's Latin chronicle "Historia Regum Britanniae".
 He also figures into Welsh genealogies.
 The name Octavius in Geoffrey of Monmouth "Historia" is a corruption and faux-Latinization of Old Welsh/Breton Outham (later spelled Eudaf).
 According to the medieval Welsh genealogy from Mostyn MS. 117, Eudaf was a direct ancestor of King Arthur.
Welsh mythology consists of both folk traditions developed in Wales, and traditions developed by the Celtic Britons elsewhere before the end of the first millennium.
 Like most predominately oral societies found in the prehistoric Britain, Welsh mythology and history was recorded orally by specialists such as druids (Welsh: "derwydd" ).
 This oral record has been lost or altered as result of outside contact and invasion over the years.
 Much of this altered mythology and history are preserved in medieval Welsh manuscripts which include the Red Book of Hergest, the White Book of Rhydderch, the Book of Aneirin and the Book of Taliesin.
 Other works connected to Welsh mythology include the ninth century Latin historical compilation "Historia Brittonum" ("History of the Britons") and Geoffrey of Monmouth's twelfth-century Latin chronicle, "Historia Regum Britanniae" ("History of the Kings of Britain") as well as later folklore such as the 1908 "The Welsh Fairy Book" by William Jenkyn Thomas.
Odhran Ua hEolais (died A.D. 994) was a medieval scribe and scholar at the abbey of Clonmacnoise.
 He must have been born, and lived his childhood, in the kingdom of "Conmaicne Magh Réin", which corresponds to present day south county Leitrim.
 We do not know any significant details of his personal life, but Odhran moved to county Offaly in adult life, to become Lector and a "famous scriba" of Clonmacnoise.
 His death is recorded in the Annals of the Four Masters.
 A cross-stone of Odhran, with his name inscription legible in middle Irish, is preserved to this day.
Bladud or Blaiddyd was a legendary king of the Britons, for whose existence there is no historical evidence.
 He is first mentioned in Geoffrey of Monmouth's "Historia Regum Britanniae", which describes him as the son of King Rud Hud Hudibras, and the tenth ruler in line from the first King, Brutus.
 A Bleydiud son of Caratauc is mentioned in the Welsh Harleian MS 3859 genealogies, suggesting to some that Geoffrey misinterpreted a scrap of Welsh genealogy (such as the Harleian genealogies or a related text).
 The Welsh form of the name is given as "Blaiddyd" in manuscripts of the "Brut Tysilio" (Welsh translations of Geoffrey's "Historia").
 The meaning of the name is "Wolf-lord" (Welsh "blaidd" "wolf" + "iudd" "lord").
 In the text he is said to have founded the city of Bath.
 He was succeeded by his son Leir (King Lear).
Cordelia is a fictional character in William Shakespeare's tragic play, "King Lear".
 She is the youngest of King Lear's three daughters, and his favourite.
 After her elderly father offers her the opportunity to profess her love to him in return for one third of the land in his kingdom, she refuses and is banished for the majority of the play.
The following list of legendary kings of Britain derives predominantly from Geoffrey of Monmouth's circa 1136 work "Historia Regum Britanniae" ("the History of the Kings of Britain").
 Geoffrey constructed a largely fictional history for the Britons (ancestors of the Welsh, the Cornish and the Bretons), partly based on the work of earlier medieval historians like Gildas, Nennius and Bede, partly from Welsh genealogies and saints' lives, partly from sources now lost and unidentifiable, and partly from his own imagination (see bibliography).
 Several of his kings are based on genuine historical figures, but appear in unhistorical narratives.
 A number of Middle Welsh versions of Geoffrey's "Historia" exist.
 All post-date Geoffrey's text, but may give us some insight into any native traditions Geoffrey may have drawn on.
Goneril is a character in Shakespeare's tragic play "King Lear" (1605).
 She is the eldest of King Lear's three daughters.
 Along with her sister Regan, Goneril is considered a villain, obsessed with power and overthrowing her elderly father as ruler of the kingdom of Britain.
 Her aggressiveness is a rare trait for a female character in Elizabethan literature.
"Duke of Cornwall" appears as a title in pseudo-historical authors as Nennius and Geoffrey of Monmouth.
 The list is patchy and not every succession was unbroken.
 Indeed, Geoffrey repeatedly introduces Dukes of Cornwall only to promote them to the Kingship of the Britons and thus put an end to their line as (merely) dukes.
 As adjuncts or supporting roles to the kings of the Britons, the legendary dukes of Cornwall are considered part of the vast Matter of Britain.
Regan is a fictional character in William Shakespeare's tragic play, "King Lear", named after a King of the Britons recorded by the medieval scribe Geoffrey of Monmouth.