Lay of Leithian
The long Elvish lay that told the story of Beren and Lúthien, their Quest for the Silmaril, and their return from Mandos. It was said to be the second longest of all such tales (with the longest being the Narn i Hîn Húrin, the story of Túrin and Nienor Níniel).
The Lay tells the story of Beren's escape from Dorthonion after the loss of his father Barahir. Coming into the south, he entered Doriath and came across Lúthien Tinúviel in the woods. They desired to wed, but Lúthien's father Thingol set an impossible bride-price on his daughter - a Silmaril from the Iron Crown of Morgoth in the deepest pits of Angband. Beren set out on his hopeless quest with the aid of Finrod Felagund, but they were captured and imprisoned by Sauron. Lúthien came to their aid through many troubles of her own, and with the help of Huan the Hound she rescued Beren. Using her magical arts, they penetrated Angband and stole one of the Silmarils, but in their escape Beren's hand, holding the Silmaril, was bitten from his wrist by the great wolf Carcharoth. Eventually, the wolf was hunted and slain, and the Silmaril recovered, but only at the cost of Beren's life. Then Lúthien, too, passed away, and pleaded before Mandos himself. Both Beren and Lúthien were returned to life, and they dwelt in the south of Ossiriand for a time. Lúthien had become mortal herself, and she passed away at last with her beloved beyond the Circles of the World.
The Lay is not a mere literary invention - it does substantially exist, and is contained within volume III of The History of Middle-earth, appropriately named The Lays of Beleriand. Though the extant lay runs to more than four thousand lines, Tolkien never fully completed the poem.