J. R. R. Tolkien was a master of Worldbuilding, working on his Middle-Earth world from about WW1 until his death. The Lord of the Rings is full of lovingly crafted and referred-to details, many of which are left unexplained, whose stories first got public with the posthumous publications of the earlier stories.
- One thing Tolkien knew from his studies as a linguist and English teacher is that some of the old myths recreate the Cryptic Background Reference effect entirely by accident, when the relevant poems or stories are lost — the medieval Finns probably had an explanation of what a Sampo (from The Kalevala) is, for example, but it didn’t survive the Middle Ages.
- Then there are some things which never got elaborated on, even posthumously, like in The Hobbit when Bilbo makes reference to “the wild Were-worms in the Last Desert.” Nothing remotely similar is ever even spoken of again.
- “Far, far below the deepest delvings of the Dwarves, the world is gnawed by nameless things.”
- Half of fun of reading Tolkien is this. Go read The Silmarillion and go back and read The Lord of the Rings. Now revel in all the references most people didn’t get the first time around. That part of the song Aragorn sings in The Fellowship of the Ring about Beren and Lúthien? Now you know the whole story. Bilbo’s song about Eärendil that Aragorn seemed to find so cheeky to sing in Rivendell? It was about Elrond’s father (and mother) who he hasn’t seen in five thousand years and probably dredged up some bad memories about the ransacking of his home when he was a child by the sons of Fëanor. The list goes on.
- The Second Prophecy of Mandos, which describes what the end of the world will be like, is referenced (though not by name) in virtually all of the canonical stories of Middle-earth. However, the prophecy itself does not appear in canon — only in Tolkien’s earlier drafts for The Silmarillion.
https://tvtropes.org/pmwiki/pmwiki.php/Main/CrypticBackgroundReference