The Finnish epic Kalevala strongly influenced J.R.R. Tolkien’s mythology and the story of Túrin Turambar.
Tolkien studied Finnish while developing his invented languages, and the structure and sound of Finnish influenced Quenya, one of his Elvish languages. He was also deeply interested in the Kalevala, a compilation of Finnish oral poetry published by Elias Lönnrot in the 19th century.
The tragic figure of Kullervo in the Kalevala was especially important to Tolkien. Tolkien’s unfinished early work “The Story of Kullervo” directly engages with that material, and elements of Kullervo’s story contributed to Túrin Turambar in the legendarium. Túrin’s tale appears in expanded form in The Children of Húrin.
Beowulf, the Poetic Edda, and Welsh traditions also influenced Tolkien, but they are different sources. The Kalevala is specifically the Finnish epic most closely associated with the Kullervo connection.