What is "fervaal"

Fervaal is an opera (action musicale or lyric drama) in three acts with a prologue by the French composer Vincent d'Indy, his opus 40. The composer wrote his own libretto, based in part on the lyric poem Axel by the Swedish author Esaias Tegnér. D'Indy worked on the opera over the years 1889 to 1895, and the score was published in 1895.