Also known as the river of light, the Effulgent Flow does what its name implies: it emits a bright glow. It forms the backbone of the Unrar as it brings light and with it, life into the underwold. The river springs high up in the mountains near Skyfall, runs through the Emerald Forest and plunges down into the Earths Mouth. The river never reaches the seas as it plummits futher into the deep via the Worldhole, far beyond human reach.
The reason this river emits light is due to its spring. Before reaching the surface, the river crawls from the deep through many layers of Adamantine which leaves sedements in the water. These sedements absorb the light of the sun and moon during its short stay at the surface. Plants drinking this fluid become luminous themselves, not more than a soft glow but enough to light the halls beneath the earth.