A contribution by Riku66
Dark river, underworld.
Tuonela is the realm of the dead or the Underworld in Finnish mythology.
According to traditional Finnish religion, the fate of good and bad people is the same and the dead wander the afterlife as shadow-like ghosts. Tuoni, god of the dead, and his wife Tuonetar are the rulers of Tuonela.
According to scholars Felix Oinas and Juha Pentikäinen, Tuonela is described as being at the northernmost part of the world, but is sectioned apart from the world of the living by a great divide. In the divide flows the dark river of Tuonela. The river is wild, and the dead can be seen trying to swim across it. The dead must cross the river, either by a thread bridge, swimming, or taking a boat piloted by the daughter of Tuoni. The river is guarded by a black swan that sings death spells.