Hi there, have you notices the similarities the plot of Beren and Luthien and Grimm brother's The Devil with the Three Golden Hairs?

The Devil with the Three Golden Hairs

A “common man” marries a princess during her father’s absence (there are much more details on how this actually happens, but those are the main facts). When the king returns and finds out, he sends his new son-in-law to the hell to come back with 3 hairs from the devil´s head, if he wishes to stay married with his daughter. Basically, he sends him to die, because he does not approve of this marriage.

The young man manages to get to hell, there he meets the devil´s grandmother (o similar figure, depending on the translation) and she decide to help the young man, so when the devil arrives, he eats and drinks and falls asleep on his grandmother's lap. She plucks three golden hair from his head and gives them to the man.

The man returns to the princess and the marriage is approved.

Beren and Luthien

Beren is human, meets Luthien in a forest and they both fall in love. When king Thingol finds out about he orders Beren to come speak with him, he does not approve of this relationship, he calls Beren “baseborn” and orders him that, if he wants Luthien’s hand, he should get a silmaril from Morgoths crown and bring it back to him.

Beren manages to get to Angband. Luthien sings a song that makes Morgoth fall asleep, this way Beren is able to get a silmaril from the iron crown.

He comes back to Thingol and he marries Luthien.

I’m sure Tolkien was fully aware of these similarities. The backbone of both stories is basically the same.