Stenkil is the first commonly accepted king of Gothenland. He also ruled Svealand following the death of his father-in-law Emund.
While the Svear tribe on the east coast of Scandinavia had coalesced into a singular kingdom during the 10th century the Geats to their south-west had done no such thing. The people though perfectly willing to defend themselves against Svear (as attested to in the Beowulf epic), Danish or Norwegian aggression, were still divided politically. Both Eric V and Olof are claimed to have some sort of over-lordship over Gothenland but in reality their authority only reached as far as the borders of Vaster- and Östergötland.
Out of this appeared Stenkil, possibly the petty king or Jarl of Vastergötland. Though no war or battle is recorded it seems he had been accepted as the Geats' king in around 1047. The then king of Svealand, Anund of Svealand accepted him too handing him his niece in marriage. Though perhaps the events were the other way round and Anund formed an alliance with Stenkil helping him come to power.
Either way it seems as if he was greatly admired and loved by his new kingdom though little of his actual deeds are recorded. His position perhaps strengthened after his father-in-law Emund became king of Svealand in 1050. Both of Emund's sons died before him and Stenkil was accepted as his successor. However the Svealandic Thing refused to allow him to 'rule the two kingdoms as one'. He himself appeared to like Gothenland much more anyway and his son Inge was soon installed as regent while Stenkil dealt with threats from Harald Hardrada to the west.
As Gothenland was much more Christianised than Svealand Stenkil's customs were not overly appreciated anyway and he cautioned Inge not to push too far too soon, preventing missionary bishops from Bremen from destroying the Svealandic temples.
Stenkil died in 1066 at which point his kingdoms were divided. His wife is only usually known as 'Ingamoder' i.e. Inge's mother and how many children they had is disputed. While Inge I is definitely his as well as his immediate successor in Gothenland, Eric I, there are other contenders. Halsten, the father of Philip and Inge II is generally assumed to be Stenkil's son. Less certain is Hakan Rode, who succeeded Olaf I, was possibly Stenkil's son too.