Sweyn travelled with Eric II to Norway in the mid-1130s, when his father fought King Niels to win the Danish throne.

[2] At the abdication of Eric III in 1146, Sweyn was elected king by the magnates on Zealand while Canute V was crowned by their counterparts in Jutland.

[3] For the next years, Sweyn fought a civil war against Canute for the kingship of Denmark, supported by Valdemar.

[2] After several battles, Sweyn conquered Funen and parts of Jutland, and set Valdemar up as Duke of Schleswig.

[2] The deal made Sweyn "premier king", with Canute eligible to receive a substantial portion of Denmark, and Valdemar keeping the Duchy of Schleswig.

Sweyn's army was defeated, and he was killed by peasants who caught him when his horse was sucked into a bog while he fled from the battle.