It is common knowledge among anyone who’s spent time wandering the history of Wales that murder and mayhem among the ruling families for power was common.

David Walker (Medieval Wales, 1990) writes:  “Early entries in the Welsh Annales are brief in the extreme, but there are hints of ugly deeds.  In 814, Griffri ap Cyngen was slain by the treachery of his brother; in 904, Merfyn ap Rhodri of Gwynedd was killed by his own men; in 969 Ieuaf ab Idwal of Gwynedd was seized by his brother Iago and imprisoned; in 974, Meurig ab Idwal was blinded” (p. 6).

He further points to the book Courtier’s Trifles by Walter Map, who satirized the reign of Gruffydd ap Llywelyn (1039-63 AD), who ruled all of Wales from 1055 until his death.  He writes:  “I name Hywel, whom you caused to be smothered in secret when he was on a mission for you; Rhydderch, whom you received with a kiss and embrace and killed with a knife in your left hand; Tewdws, whom as he walked and talked with you, you tripped up with your foot and cast down the sheer rocks; and your nephew Meilin, whom you secretly captured by guile and let him die loaded with chains in your dungeon.” (p. 6)

This entry from Wikipedia is classic:  “Hywel ap Ieuaf (died 985) was a King of Gwynedd in north-west Wales from 979 to 985.

Hywel was the son of Ieuaf ap Idwal who had ruled Gwynedd jointly with his brother Iago ab Idwal until 969. In that year the sons of Idwal quarrelled and Iago took Ieuaf prisoner . . . In 974 Hywel raised an army and drove his uncle from Gwynedd temporarily. Iago was able to return, but was forced to share power with his nephew. In 978 Hywel made another attempt to take the kingdom from his uncle, raiding the monastery at Clynnog Fawr. In this raid Hywel was helped by English troops, possibly provided by Aelfhere, Earl of Mercia. Hywel defeated Iago in battle in 979, and the same year Iago was captured by a force of Vikings, possibly in Hywel’s pay, and vanished from the scene. Hywel was left as sole ruler of Gwynedd, but apparently did not set his father free, since according to J.E. Lloyd, Ieuaf remained in captivity until 988.

In 980 Hywel faced a challenge from Iago’s son, Custennin ab Iago, who attacked Anglesey in alliance with Godfrey Haraldsson, a Viking chief from the Isle of Man. Hywel defeated them in battle, killing Custennin and putting Godfrey and his men to flight. Now securely in possession of Gwynedd, Hywel aimed to expand his kingdom to the south. He again made an alliance with Aelfhere of Mercia and attacked Brycheiniog and Morgannwg with some success, although he was not able to annex these kingdoms. However in 985 his English allies turned on him and killed him, possibly alarmed by his growing power. He was succeeded by his brother Cadwallon ap Ieuaf, who had not been on the throne long when Gwynedd was annexed by Maredudd ap Owain of Deheubarth.”  http://en.wikipedia.org/wiki/Hywel_ab_Ieuaf

This is much like the open warfare that erupted among the sons of Owain Gwynedd upon his death in 1170:  his youngest sons, Dafydd and Rhodri (children of Cristina), systematically eliminated all of their rivals, including Hywel, the eldest (d. 1170).  “Dafydd drove out Maelgwn in 1173, sending him fleeing to Ireland.  Another brother, Cynan, died in 1174, removing one more contender for the throne. The same year Dafydd captured and imprisoned his brothers Maelgwn (who had returned from Ireland) and Rhodri.”  http://en.wikipedia.org/wiki/Dafydd_ab_Owain_Gwynedd

However, Llywelyn ap Iowerth (son of another dead brother) overthrew his uncle in 1294 and claimed the throne for himself.  Thus began the remarkable rule of Llywelyn Fawr.

Other examples:

Gruffydd ap Llywelyn (d 1244) did everything in his power to undermine the rule of his brother, Dafydd.  Dafydd retaliated by imprisoning him at Criccieth Castle.  Gruffydd ultimately fell to his death trying to escape the Tower of London in which he’d been imprisoned by Henry III.

Llywelyn ap Gruffydd imprisoned his brother, Owain from 1256 to 1277 to prevent him from vying for the throne of Gwynedd.  Their younger brother, Dafydd, defected twice to the English crown and tried to assassinate Llywelyn in 1274.