The choice of Luthien
"But the other choice was this: that she might return to Middle-earth, and take with her Beren, there to dwell again, but without certitude of life or joy. Then she would become mortal, land subject to a second death, even as he; and ere long she would leave the world for ever, and her beauty become only a memory in song.
This doom she chose..."
J.R.R.Tolkien, "The Silmarillion"