At the ending of the Quest for the Silmaril, Beren met his death at the jaws of the great wolf Carcharoth. So great was Lúthien's love of him, though, that she surrendered her own immortality so that he might return to Middle-earth for a time.
They returned out of the Halls of Waiting, and on the green island of Tol Galen in the southern lands of Ossiriand, they lived in a Land of the Dead that Live, or Dor Firn-i-Guinar in the Elven tongue.
No mortal Man saw Beren or Lúthien ever again, but they had happy relations with the Green-elves of Ossiriand who lived around them. After the slaying of Lúthien's father Thingol, Beren took a force of Green-elves north to avenge his death, and recover the stolen Silmaril.
From that time, the Silmaril was kept in Dor Firn-i-Guinar and borne by Lúthien, and that land was said to become 'like a vision of the land of the Valar'. Dior, the son of Beren and Lúthien, then left Dor Firn-i-Guinar with his family, and went to take up the rule his lost grandfather's kingdom of Doriath.
Both Beren and Lúthien were mortal, and at last their lives came to an end, and with them their Land of the Dead that Live. A lord of the Green-elves took the Silmaril, and brought it to their heir, Dior, in Doriath, and Dor Firn-i-Guinar was no more.