Question: What is the name of the king who ruled in the South West of England, and became the ruler of Mercia upon the death of his sister?

Context articles: South West Observatory
Sighere of Essex
Edward the Elder
Akaba of Dahomey
Ceolwulf II of Mercia
Tribute South West 1 West
Æthelred, Lord of the Mercians
Winchcombeshire
South West Combination Women's Football League
Æthelflæd

The South West Observatory (SWO) is a regional resource for the South West of England, originally established by the former South West Regional Development Agency, Government Office for the South West, and the South West Regional Assembly (now South West Councils) as a partnership for use by policy-makers to aid and improve evidence-based decision-making.
Sighere was the joint king of the Kingdom of Essex along with his cousin Sæbbi from 663/4 to about 688.
 He was son of Sigeberht Sæwarding, probably Saint Sigeberht, but perhaps Sigeberht the Little.
 He was outlived by Sæbbi, who became the sole ruler of Essex after his death.
 Sighere and Sæbbi were cousins of their predecessor Swithelm.
 While Sighere returned to paganism, Sæbbi remained Christian.
 They soon developed a rivalry.
 Sighere found an ally in Wessex, and Sæbbi with Mercia.
 As a result of their rivalry, King Wulfhere of Mercia established himself as overlord of Essex.
 and induced Sighere to marry his niece Osgyth, daughter of Frithuwold, sub-king of Mercia in Surrey.
 Jaruman, the bishop of Mercia, was assigned to reconvert the people of Essex to Christianity.
 In 673, Sighere separated from Osgyth, who then fled to the protection of Bishop Beaduwine of North Elmham.
 Sighere died about 688, and Sæbbi became sole ruler of Essex.
Edward the Elder (Old English: "Eadweard cyning "; c. 874 – 17 July 924) was King of the Anglo-Saxons from 899 until his death.
 He became king in 899 upon the death of his father, Alfred the Great.
 He captured the eastern Midlands and East Anglia from the Danes in 917 and became ruler of Mercia in 918 upon the death of Æthelflæd, his sister.
Akaba was an early King of the Kingdom of Dahomey, in present-day Benin, from 1685 until "c."1716.
 King Houegbadja had created the basic structure of the kingdom on the Abomey plateau.
 His first children were the twins of Akaba and Hangbe and they were followed by another son of Houegbadja who would become King Agaja.
 As the oldest son, Akaba became the king upon Houegbadja's death and ruled until 1716 when he died during battle in the Ouémé River Valley, either of small pox or in battle.
 When he died his sister, Hangbe, became the ruler and began preparing Akaba's oldest son, Agbo Sassa, for the throne.
 In 1718, Agaja, the next oldest son after Akaba from Houegbadja, fought with Agbo Sassa and Hangbe and became the next King of Dahomey.
Ceolwulf II (died c. 879) was the last king of independent Mercia.
 He succeeded Burgred of Mercia who was deposed by the Vikings in 874.
 His reign is generally dated 874 to 879 based on a Mercian regnal list which gives him a reign of five years.
 However, D. P. Kirby argues that he probably reigned into the early 880s.
 By 883, he had been replaced by Æthelred, Lord of the Mercians, who became ruler of Mercia under the lordship of Alfred the Great, king of Wessex.
Tribute South West 1 West is an English, level six, rugby union league for clubs based in the south-west of England; mainly Bristol, Cornwall, Devon, Gloucestershire and Somerset.
 The champions are promoted to South West Premier (formerly National League 3 South West) and the runner-up plays the second team in South West 1 East, with the winning team gaining promotion.
 Relegated sides usually play in one of the three seventh-tier leagues — Tribute Western Counties North, Tribute Western Counties West or occasionally, Southern Counties South.
 The current champions are Weston-super-Mare and they will play in South West Premier for season 2017–18.
Æthelred, Lord of the Mercians (or Ealdorman Æthelred of Mercia; died 911) became ruler of English Mercia shortly after the death of its last king, Ceolwulf II in 879.
 His rule was confined to the western half, as eastern Mercia was then part of the Viking-ruled Danelaw.
 Æthelred's ancestry is unknown.
 He was probably the leader of an unsuccessful Mercian invasion of Wales in 881, and soon afterwards he acknowledged the lordship of King Alfred the Great of Wessex.
 The alliance was cemented by the marriage of Æthelred to Alfred's daughter Æthelflæd.
Winchcombeshire was an ancient county in the South West of England, in the Anglo-Saxon period, with Winchcombe as its county town.
 The county originated in the shiring of Mercia in the tenth or early eleventh centuries, perhaps by King Edward the Elder in the early 920s.
 It was merged into Gloucestershire in the early eleventh century, probably by King Cnut in 1017.
The South West Combination Women's Football League, also known simply as the South West Combination, was a women's football league in England from 1998 to 2014.
 Until the creation of the FA Women's Super League in 2011, they sat at the third level of English women's football pyramid along with the three other Combination Leagues – South East, Midland and Northern.
 The South West Combination fed into the FA Women's Premier League Southern Division, and lied above the Southern Region Women's Football League and the South West Regional Women's Football League in the pyramid.
 For the 2014–15 season the South West Combination was incorporated into the newly re-structured FA Women's Premier League as the FA WPL South West Division One.
Æthelflæd, Lady of the Mercians ( 870 - 12 June 918) ruled Mercia in the English Midlands from 911 until her death.
 She was the eldest daughter of Alfred the Great, king of the Anglo-Saxon kingdom of Wessex, and his wife Ealhswith.
 Æthelflæd was born around 870 at the height of the Viking invasions of England.
 By 878 most of England was under Danish Viking rule, East Anglia and Northumbria having been conquered and Mercia partitioned between the English and the Vikings, but in that year Alfred won a crucial victory at the Battle of Edington.
 Soon afterwards the English-controlled western half of Mercia came under the rule of Æthelred, Lord of the Mercians, who accepted Alfred's overlordship.
 Alfred adopted the title King of the English, claiming to rule all English people not living in areas under Viking control.
 In the mid-880s, Alfred sealed the strategic alliance between the surviving English kingdoms by marrying Æthelflæd to Æthelred.