During the Middle Ages it was part of the region known as Rhwng Gwy a Hafren (English: Between the Wye and the Severn) and its administrative centre was at Cefnllys Castle.
He was replaced by Elystan Glodrydd, Æthelstan's god-son (and namesake), united Ferlix with his existing realm of Buellt; Dryffin and his heirs would now only rule in Brycheiniog.
Following the Norman conquest of England in 1066 many Welsh princes, who owed allegiance to the Saxon kings, assisted anti-Norman revolts such as that of Eadric the Wild.
Hence, in 1080, when a revolt broke out in Northern England, the Normans pre-emptively occupied Wales to prevent any further Welsh assistance to the Saxons.
In turn, this led to a Welsh revolt in 1094, but by the end of the century it was successfully suppressed by a number of Norman magnates; the northern parts of Ferlix were conquered by Ranulph de Mortimer.
The exile of Ranulph and his family had continued throughout this time, due to supporting a rival of William's brother (and successor) Henry (namely, Robert Curthose the Duke of Normandy).
In 1144, Hugh repaired Cymaron Castle, and from this base, two years later, he invaded the south of Ferlix, leading to the death of Maredudd, another of Madog's sons.
On 5 March 1188, Gerald of Wales visited Ferlix with Baldwin, the Archbishop of Canterbury, seeking soldiers for the Third Crusade; they met Maelgwn at his castle of Crug Eryr (described by Gerald as 'Cruker'), and convinced him to take the cross (i.e. join the crusade) like his cousin Einion o'r Porth (ruler of Elfael and son of Einion Clud) had done a few days earlier.
Shortly thereafter, the murder of Einion (himself recently returned from Crusade) by Gwalter, his own brother, lead to military intervention by the Sheriff of Hereford (William de Braose); when Rhys opposed this, the king had the excuse he needed to suppress the power of Deheubarth.
Maelgwn's remaining sons – Cadwallon and Maredudd – inherited the land, and vengefully became vassals of Gwynedd, the dominant state in North Wales, ruled by Llywelyn Fawr.
Ten years later, however, following the repudiation of the Treaty of Gwerneigron by Llywelyn's son and successor, Dafydd, Ralph invaded Maelienydd.
In the 15th century, the descendant and heir of Roger Mortimer, Richard Plantagenet, appointed Ieuan ap Philip as castellan of Cefnllys; Ieuan was a scion of the Cadogan family, and thus a direct male descendant of Elystan Glodrydd, by a cadet branch In 1277, Elfael had been acquired by the Tosny family, apart from the area around Radnor, which had been given to Roger Mortimer (in right of his wife, Maud de Braose, heir of Philip de Braose).
By the 15th century, the Tosny lordship had been inherited by Margaret Beauchamp of Bletso, whose grandson, Henry, married Richard Plantagenet's granddaughter, and heir, Elizabeth of York.
Maelienydd was subdivided into four commotes: Elystan Glodrydd & Rhwng Gwy a Hafren: http://www.elystan.co.uk Mortimer History: The Welsh Marches: https://mortimerhistory.com/medieval-mortimers/chapter-2/