Tarn Aeluin, sometimes referred to as just Aeluin, was a clear blue mountain lake in the eastern highlands of Dorthonion, southwest of Ladros. It was situated among wild heather hills, themselves surrounded by trackless and uninhabited forests and highland plains of heather. Alder-trees were known to grow beside the lake.
Tarn Aeluin was said to have been hallowed by Melian the Maia.
When Aegnor saw the reflection of Andreth in Tarn Aeluin, he fell in love with her.
Barahir and his twelve followers hid from Morgoth's forces after the Dagor Bragollach and made their lair at Tarn Aeluin. There they remained hidden until they were betrayed by Gorlim.