River of Souls

The River of Souls is a river that carries the souls of the dead. When people die, their souls leaves the physical body and is drawn to the River by the soul altar. According to the book soul journey, written by Aretha, the River then leads the souls through "unchartable realms, transcending the plane of the world." Cerberus, a three-headed hellhound, is tasked to be its Guardian, while the Key Master keeps her from escaping.

According to the same book, the Dark Altar found in Great Kourend's Arceuus House is responsible for creating the power of their soul altar, drawing energy from the real Soul Altar, whose location is unknown. Without it, the souls of Great Kourend would not be able to reach the River as easily.

Parts of the river is can be seen from the soul altar and in Cerberus' Lair.

Trivia

 * The River of Souls is likely a reference to the, which serves a similar purpose.