Battle on the Ice of Lake Vänern
The Swedish king Ohthere (Ottar Vendelkråka, who is often called the first historical king of Sweden) had died and his brother Onela (Áli) had usurped the Swedish throne. Ottar's sons Eanmund and Eadgils (Adils) had to flee to Geatland and seek refuge with the Geatish king Heardred. This induced Onela to attack the Geats and kill both Eanmund and Heardred. In order to avenge his king and kinsman, Beowulf decided to help Eadgils gain the throne of Sweden. During the battle Eadgils slew Onela and became the king of Sweden.
Norse sagas 
In the Norse sagas, some changes have appeared. Onela's name Áli hinn upplenzki "Onela of Uppland" was preserved but, Uppland, the core province of the Swedes had been confused with its Norwegian namesake Oppland, and so Onela was referred to as a Norwegian king.
In the second account, Snorri relates: They decided to fight on the ice of the water which is called Vänern... In this fight king Áli died and a great many of his people. Then king Adils took from him his helmet Hildisvín (battle-boar) and his horse Hrafn.
In the Ynglinga saga, Snorri relates that King Adils (Eadgils) fought hard battles with the Norwegian king who was called Áli hin upplenzki. They fought on the ice of Lake Vänern, where Áli fell and Adils won. Snorri relates that much is told about this event in the saga of the Sköldungs, and that Adils took Hrafn (Raven), Áli's horse.
The Skjöldunga saga is lost but at the end of the 16th century, Arngrímur Jónsson saved a piece of information from this saga in Latin. He wrote: There was animosity between king Adils of Sweden and the Norwegian king Áli of Uppland. They decided to fight on the ice of Lake Vänern. Adils won and took his helmet, chainmail and horse.
The accounts of the Battle on the Ice contain accurate information about this time and the Swedish Vendel Age. This period was characterized by the appearance of mounted warriors who fought on horseback and by the use of boar-crested helmets.
In the Battle on the Ice, the combatants are described as fighting on horseback, although the later Vikings and Anglo-Saxons who told of this battle in their legends would fight on foot. Likewise, Onela's helmet is called the battle-boar although the boar-crested helmets were long out of use by the time records of the event were written down.