Complex sentence: It is considered the greatest Finnish epic poem outside the native Kalevala tradition and contains tales of the Finnish War of 1808-09 with Russia, in which Sweden ignominiously lost Finland, which became an Arch-Duchy in the Russian empire.
Split into simpler sentences: It is considered the greatest Finnish epic poem outside the native Kalevala tradition and contains tales of the Finnish War of 1808-09 with Russia. In the war, Sweden ignominiously lost Finland, which became an Arch-Duchy in the Russian empire.