Kullervo's tale is just one of 50 songs in the Kalevala, an epic of 22,795 verses telling the story of the Sampo, a magical object that bestows power on whoever possesses it. Tolkien used numerous plot elements from the Kalevala in his own novels - a powerful magical object, incest, battles between brothers, and orphan heroes setting out on quests.
"Kullervo is the origin story for Shakespeare's Hamlet - a young man whose uncle kills his father and on whom he wreaks a terrible vengeance," says Verlyn Flieger. "It is likely that Tolkien knew that Shakespeare had used this tale."
In The Silmarillion (begun in 1914, but only published after his death), Tolkien turns Kullervo into Turin Turambar, the warrior hero.
"I think he liked the Kalevala because it has both high and low elements," suggests Tolkien-biographer Prof John Garth from the University of Nevada. "There are clodhopping idiots, treated in a really down-to-earth, anti-heroic way. In Tolkien's own fiction, he creates totally different moods. The hobbits are very relatable, very friendly; and then the elves are much more remote."
a tip of the hat to Horace