J.R.R. Tolkien’s unfinished posthumous mythology published by Christopher Tolkien in 1977 is The Silmarillion.
The book presents stories from the creation of Arda through the First Age and beyond. Its central narrative concerns the Silmarils, three jewels made by the Elf Fëanor, and the wars caused by their theft by Morgoth. The volume also includes the creation account ‘Ainulindalë,’ the shaping of the world in ‘Valaquenta,’ and later histories of the Elves, Men, and major conflicts.
J.R.R. Tolkien worked on related materials for decades and repeatedly revised them. He did not complete a definitive final version before his death in 1973. Christopher Tolkien therefore selected, arranged, and edited texts from his father’s papers, sometimes combining different versions to create a coherent published book.
The Silmarillion is not a conventional sequel to The Lord of the Rings. It is more like a mythological and historical foundation for Tolkien’s invented world, with a denser style and many generations of characters.