Kullervo's was the most tragic of the Kalevala's stories, and one of the bleakest in any heroic tradition. He was the son of Kalervo, who was killed in a feud by his brother Untamo before Kullervo's birth; Kullervo was born in slavery to Untamo, who tried more than once to kill the dangerous infant — putting him in a barrel and rolling him into the sea; tying him to a birch and burning the tree; throwing him into a swamp — but Kullervo survived every attempt with uncanny good luck, growing up an enslaved boy of monstrous strength.

Untamo at last sold Kullervo to the smith Ilmarinen, in whose house Kullervo worked as a slave for the smith's wife. The wife treated him cruelly; she baked a stone into his bread, that he should break his teeth on it; he in revenge sang her cattle into bears, who tore her to pieces. Then Kullervo fled, learned that his parents had escaped Untamo and lived in hiding, was reunited with his mother and father and brothers and sisters. But on a journey home he met a young woman in a forest and seduced her; she revealed in time that she was his own sister, who had been lost as a child. The girl drowned herself in the river; Kullervo went to take vengeance on Untamo, killing his uncle and all his uncle's people; and afterward, finding when he returned home that his mother, father, brother, and sister had all died, he came to a familiar field, drew his sword, asked it whether it was willing to drink the blood of an evil man, and threw himself upon it. The sword answered "yes," and Kullervo died.

The figure is the bleakest in Finnish heroic poetry — a child of slavery and feud who can do nothing but destroy, including himself — and J. R. R. Tolkien acknowledged Kullervo as a major influence on the figure of Túrin Turambar in The Silmarillion, with the incest-with-sister-and-suicide motif preserved nearly intact. The Kullervo-songs are among the oldest layers of the Kalevala material, with parallels in Estonian, Karelian, and other Finnic traditions, suggesting that the underlying tale is much older than the nineteenth-century compilation.