The libretto of Mlada is by Rimsky-Korsakov based on materials by Gedeonov and Krilov. The story is outlined in the leaflet of Naxos 8.550486: Mlada, a dream-figure, is betrothed to Yaromit, but at her wedding is murdered through a poisoned ring, given her by Voyslava, daughter of a prince who wishes to bring about Yaromir's downfall, a devotee of the infernal goddess Morena. The intervention of the spectral Mlada prevents the embraces of Yaromir and Voyslava, who is eventualy killed by the man she had hoped to deceive and claimed by the goddess Morena.
See also a sinopsis of Mlada at this site.The four-acts Opera-ballet Mlada composed by Rimsky-Korsakov in 1889-90 is a complete, published, recorded and performed work world-wide. The libretto is by the composer.