The Norse in Scandinavia were sick and tired of the shortage of farming land and a population boom.
So Norse traders traded with the Anglo Teutons in Britain and relized the power of the Teutons.
And they also traded with the Slavs in Russia and saw how they were not so strong.
Then it happend. The Norse had raided Novgorod. For years the Norse had raided Slavic settlements in Russia.
Then the Norse became more abtioinus. They invaded Novgorod and captured it. Many Russians were enslaved and the Norse burned down Nogorod and Norse warriors broght their families. Over time Novgorod became a Norse city and the land around it became Norse farms and villiges home to Norse clans who used to rival and then became friends in Russia. Only Novgorod and the land around it was Norse. The rest of Russia was Slavic.