Lúthien gave up his elf identity for Belem and became a mortal. Later, Belem died in battle, and Lúthien died of grief.
One of their two great-grandchildren chose to be the ancestor of mankind, Elros and Aragon; One who continues to be an elf is Master Ellund, the father of Evan.
If you want to know more details, please see here/view/1318221.htm.
Ps: The song upstairs was sung by Aragon at the coronation of the third movie, but the minor in the first movie was not included in the soundtrack, which means that this song is only a paragraph in the movie, and it is probably sung casually.