Arvernien was a coastland region to the west of Sirion. It came to prominence late in the First Age when many people fled to the coast after the Noldor were overrun in the north.

After the havens at Eglarest and Brithombar were destroyed, many of Círdan’s people fled to Arvernien and dwelt in the Forest of Nimbrethil.

“Eärendil was a mariner
that tarried in Arvernien
he built a boat of timber felled
in Nimbrethil to journey in”

The Song of Eärendil by Bilbo Baggins