The Power of the Fiend (Russian: Вражья сила, Vrazhya sila) is an opera in five acts by Alexander Serov, composed during 1867-1871.

The opera was premiered posthumously on 19 April 1871 (Old Style) at the Mariinsky Theatre in Saint Petersburg conducted by Eduard Nápravník.

If not for the delay caused by the rift with Ostrovsky, Serov might have completed the opera, but unfortunately the music of the final act was lacking at the time of his death.

Despite a new edition of the opera completed by Boris Asafiev for a Soviet performance in 1947, The Power of the Fiend still remains well outside of the repertory.

(Note: Due to gaps in information from one source to another, this section combines data from the play, the available piano-vocal score, and Bernandt, all cited in the bibliography below.)

After Il'ya goes off to attend vespers, Dasha tries to win back Pyotr's love, but he rejects her (he has become attracted to Grunya, the innkeeper's daughter) and leaves once again.

As the others depart, Spiridonovna, desiring that her daughter marry someone well-off, urges Grunya to use her wiles on Pyotr the next time he comes in.

Out in the ravine, near a weakly lighted hut, amidst noises of a nocturnal snowstorm, Pyotr imagines voices of the previous few hours.

The sound of approaching sleighbells frightens Yeryomka away: Vasya could not find Dasha to warn her, and so he, along with Il'ya and her parents, have driven to the site of the planned murder, only to come upon Pyotr, who is condemned by his father.