"Then he sacrificed his eighth son, and lived thereafter ten years, lying in his bed. Now he sacrificed his ninth son, and lived ten years more; but so that he drank out of a horn like a weaned infant. He had now only one son remaining, whom he also wanted to sacrifice, and to give Odin Upsal and the domains thereunto belonging, under the name of the Ten Lands, but the Swedes would not allow it; so there was no sacrifice, and King On died, and was buried in a mound at Upsal.
The bull turned round suddenly, and the king [Egil] struck him with his spear; but it tore itself out of the wound. The bull now struck his horn in the side of the horse, so that he instantly fell flat on the earth with the king. The king sprang up, and was drawing his sword, when the bull struck his horns right into the king's breast. The king's men then came up and killed the bull. The king lived but a short time, and was buried in a mound at Upsal.
King Adils was at a Disa sacrifice; and as he rode around the Disa hall his horse' Raven stumbled and fell, and the king was thrown forward upon his head, and his skull was split, and his brains dashed out against a stone. Adils died at Upsal, and was buried there in a mound."
Snorri's Ynglinga Saga on the three kings who according to legend are entombed in the Great Mounds at Uppsala.
Contemporary scholars doubt these accounts of the kings of old, and even the historicity of claiming that there were Iron Age kings who ruled the whole of Sweden has been put into question. As our contemporary notion of kingship is based on the medieval Christian view of the King as a divinely mandated ruler, ruling over a centralized kingdom, while the kings of the Germanic tribes were more or less band leaders or chieftains, the kingship claimed by the kings at Uppsala was probably based on something else than geopolitical power. My personal guess is that the Uppsala kingship was a religious title of great symbolical value. A title which was hotly contested and ultimately claimed by powerful Christian families from East and West Geatland, which during medieval times used the mandate gained by it to form the centralized kingdom of Sweden.