Niels succeeded his brother Eric Evergood and is presumed to have been the youngest son of King Sweyn II Estridson.

Niels was killed in an ensuing civil war and succeeded by Eric II Emune.

[1] He carried through reforms of his personal hird retinue, separating it into a military and an administrative branch[5] responsible for tax collection.

[1] Niels supported the canonization of his brother Canute IV the Holy by bestowing gifts on the clergy of Odense as he sought to enlarge the power and influence of the monarchy through the aid of the church.

[7] After 26 years of internal peace, conflict erupted between Magnus and his cousin Canute Lavard.

Niels and Magnus landed at Fotevik Bay in Scania in June 1134 seeking to deal Eric a decisive defeat.

At the Battle of Fotevik on 4 June 1134, Niels and Magnus were taken by surprise by a contingent of German soldiers on horses.

[2] Niels sought refuge with Holy Roman Emperor Lothar III of Germany, but did not make it past the city of Schleswig.