Question: Who was the brother of Tostig Godwinson that overthrew Gruffydd, Rhiwallon, and Bleddyn?

Context paragraphs:
Battle of Fulford: The Battle of Fulford was fought on the outskirts of the village of Fulford near York in England, on 20 September 1066, when King Harald III of Norway, also known as "Harald Hardrada" ("harðráði" in Old Norse, meaning "hard ruler"), and Tostig Godwinson, his English ally, fought and defeated the Northern Earls Edwin and Morcar.

Tostig Godwinson: Tostig Godwinson ( 1026 – 25 September 1066) was an Anglo-Saxon Earl of Northumbria and brother of King Harold Godwinson.  After being exiled by his brother, Tostig supported the Norwegian king Harald Hardrada's invasion of England, and was killed at the Battle of Stamford Bridge.

Gyrth Godwinson: Gyrth Godwinson (Old English: Gyrð Godƿinson) ( 1032 – 14 October 1066) was the fourth son of Earl Godwin, and thus a younger brother of Harold Godwinson.  He went with his eldest brother Swegen into exile to Flanders in 1051, but unlike Swegen he was able to return with the rest of the clan the following year.  Along with his brothers Harold and Tostig, Gyrth was present at his father's death-bed.

Judith of Flanders (died 1095): Judith of Flanders, Countess of Northumbria, and later Duchess of Bavaria (1030-35 to 5 March 1095) was the wife firstly of Tostig Godwinson, Earl of Northumbria, and secondly of Welf I, Duke of Bavaria.  Her niece was Matilda of Flanders, Queen consort of William the Conqueror, who was Judith's cousin.

Bleddyn ap Cynfyn: Bleddyn ap Cynfyn (Old Welsh: "Bledẏnt uab Kẏnỽẏn" ;   1073), sometimes spelled Blethyn, was an 11th-century Welsh king.  He was installed by Harold and Tostig Godwinson as king of Gwynedd in 1063 on his father's death, during their destruction of the kingdom of Bleddyn's half-brother, Gruffydd ap Llywelyn.  He became king of Powys on his brother Rhiwallon's death in 1069.  His descendants continued to rule Powys as the House of Mathrafal.

Battle of Stamford Bridge: The Battle of Stamford Bridge took place at the village of Stamford Bridge, East Riding of Yorkshire, in England on 25 September 1066, between an English army under King Harold Godwinson and an invading Norwegian force led by King Harald Hardrada and the English king's brother Tostig Godwinson.  After a bloody battle, both Hardrada and Tostig along with most of the Norwegians were killed.  Although Harold Godwinson repelled the Norwegian invaders, his army was defeated by the Normans at Hastings less than three weeks later.  The battle has traditionally been presented as symbolising the end of the Viking Age, although major Scandinavian campaigns in Britain and Ireland occurred in the following decades, such as those of King Sweyn Estrithson of Denmark in 1069–1070 and King Magnus Barefoot of Norway in 1098 and 1102–1103.

Eadulf Rus: Eadulf Rus (fl.  1080) was an 11th-century Northumbrian noble.  He was either the son or grandson of Gospatric (son of Uhtred the Bold), possibly the man who soon after Christmas 1064 was allegedly killed on behalf of Tostig, Earl of Northumbria.  This murder by Tostig led to a great northern revolt against Edward the Confessor, a revolt that turned both King Edward and Harold Godwinson against Tostig and led to the appointment of the Mercian, Morcar, as Earl of northern England.

Manor of Hougun: The Manor of Hougun is the historic name for an area which now forms part of the county of Cumbria in north-west England.  Of the three most northern counties of England surveyed in the Domesday Book of 1086 (Northumbria, Durham and Cumbria), only the southern band of land in the south of Cumbria was recorded.  The westernmost entries for Cumbria, covering the Duddon and Furness Peninsulas are largely recorded as part of the "Manor of Hougun".  The entry in Domesday Book covering Hougun refers to the time (ca. 1060) when it was held by Tostig Godwinson, Earl of Northumbria.

Rhiwallon ap Cynfyn: Rhiwallon ap Cynfyn (c. 1020 – c. 1069 ) was a Welsh King.  the son of Cynfyn ap Gwerstan and brother of King Bleddyn of Powys.  Through his mother Angharad, he was half-brother to King Gruffydd ap Llywelyn as well.  Following the 1063 invasion of Wales by Harold and Tostig Godwinson that overthrew Gruffydd, Rhiwallon and Bleddyn jointly received Powys and Gwynedd on condition of faithfully serving Edward the Confessor "everywhere by water and by land".

St Wilfrid's Church, Halton-on-Lune: At the time of the Norman conquest of England in 1066, Halton was the centre of a large manor that belonged to Earl Tostig Godwinson.  There is evidence of a Christian site at Halton and there was probably an Anglo-Saxon minster there.  There may have been a connection to Bishop Wilfrid ("c".  633 – "c".  709) and certainly by 1252, there was a church dedicated to St Wilfrid.  Until the early 18th century, the advowson (the right to appoint a parish priest) belonged to the Lord of the Manor.

Supporting paragraphs: Rhiwallon ap Cynfyn Tostig Godwinson
Answer: King Harold Godwinson