Eglė the Queen of Serpents (Žilvinas)

In variants summarized in the supplied sources, the narrative opens with Eglė bathing at a lake or sea-lagoon (marios) where she finds a coiled grass-snake (žaltys/žalčius). The snake demands that she pledge herself in exchange for the return of her clothing; she agrees and becomes the serpent's wife. The male serpent identifies himself as Žilvinas, escorts her to a palace beneath the sea (reached via a nearby island and an underground tunnel), and establishes an otherworldly household and family with her. The tale functions as an aetiology: after betrayal and the serpent's death, Eglė transforms her children and eventually herself into trees whose species-names correspond to the characters' names, explaining local arboreal identities.

Sources present the being in two primary registers: as a coiled grass-snake (žaltys/žalčius) at the water's edge that speaks and as a handsome man—described in summaries as the grass-snake prince—when greeting Eglė and escorting her to the underwater palace. The serpent's principal domain is an underwater or subterranean palace beneath a sea-lagoon; visual depictions and theatrical/sculptural commemorations emphasize a royal, otherworldly household rather than a simple animal lair. The lexical term žalčius is ambiguous in the sources and may refer to a terrestrial grass snake or evoke an aquatic/mithical serpent (possibly comparable to an eel) in the tale's seascape.

Documented tale actions attribute to Žilvinas speech, intelligence, the capacity to take a human wife and father children, rule an underwater palace, impose conditions and tasks on his bride, and teach a secret chant by which he may be summoned from the sea. He sets tests (three tasks in motifs) and prescribes limits on Eglė's surface visits. The tale also shows that, once summoned ashore by the chant and brought to land, he can be attacked and killed by humans in the narrative variant provided. The story includes metamorphic magic enacted by Eglė (a whispered enchantment) that turns humans into specified tree species, but explicit ritual formulas or wider destructive powers for Žilvinas beyond those narrative capacities are not recorded in the supplied sources.