The Battle of Leuven was fought in September 891 between the Franks and the Vikings, essentially ending the Viking invasions in the Low countries. The existence of this battle is known due to the Annales Fuldenses.
The Frankish force was led by Arnulf of Carinthia. The Franks successfully repelled the Viking assault and it is reported that the bodies of dead Northmen blocked the run of the river (ita ut cadaveribus interceptum alveum amnis siccum appareret).
After the success, Arnulf built a castle on a small island in the Dijle River.
The Annales Fuldenses, the source that mentions this battle is the first source that mentions the city of Leuven (French: Louvain), which was at that time called Loven.