A tone poem, revised 1902. In the 1940:s Sibelius told his secretary: "En Saga is an expression of a state of the soul. At the time when I was composing it I had several harrowing expreriences. In no other work have I revealed myself so completely. Merely for this reason, all literary interpretations are quite alien to me".
If you must look for a folkloristic basis for En Saga, then the mood of the tone poem is certainly closer to the Edda than to the Kalevala.
Penguin Guide 3 stars. Gramophone: recommended.