Soulmates found one story at a time.

Tuonela is the Finnish Underworld, run by Tuoni and Tuonetar. The souls of the dead are brought by soul birds, swans known as sielulintu. 

The living sometimes try to sneak in to steal the secrets of the dead and magical knowledge. There are wild rivers, whirlpools, hills, and black swans who sing death spells, and Surma who try to keep them out.