Author has written 5 stories for Silmarillion, and Chronicles of Narnia.
Disclaimer: I do not own any of the characters that were created by J.R.R. Tolkien or C.S. Lewis. I'm not making any profit by this writing, it is done purely for my own pleasure (and hopefully for others' entertainment).
I created an original character as my user identity at a Tolkien discussion board. She's an Elf, and her name is Barufiniel (or Varnafindë, which is Quenya, while Barufiniel is Sindarin). I've since developed her family into a small tree of ancestors, who may or may not appear in stories at this site. Rameldir - who has appeared - is her father. Arindis is Rameldir's mother. Calimon is Rameldir's friend, and Rameldir later marries the sister of Calimon's son-in-law.
For earlier generations I've invented fictitious siblings and children of Tolkien's existing characters, in such cases where the total number of children was not specified, and it was not otherwise obvious that there would be no more siblings. Arindis is one of those. Thus, my original characters are descended from some of the known characters from The Silmarillion.
Why then am I mentioning C.S. Lewis at all? He wrote about Narnia, didn't he, not about Elves and Silmarils?
Well, there is also cross-over ...
When I state that one of my favorite authors is "Celegorm and Curufin" (at http://www.fanfiction.net/u/1539421/Celegorm_and_Curufin ), I should also add that on that account I am Celegorm, writing with a friend.
One of my reviewers of The Treacherous Ice said, "Can't really blame anyone but the Ice or possibly Feanor or possibly even the Valar!" This reminds me of a great painting of Fingolfin coming to Middle-earth, "ready to throttle the first Feanorean to cross his path", in the words of the artist, Jenny Dolfen. He certainly looks like it.
I'm not able to link to the picture here, but search (or ask me) for "The Coming of Fingolfin".