In Slovakian folklore, Jarilo is the god of war, vegetation, fertility, spring, and the harvest. Morena is the goddess of the harvest, witchcraft, winter, and death. Jarilo is associated with the moon and Morena is considered a daughter of the sun. Both of them are children of Perun (the god of thunder and lightning).
They are born on the night of the new year, but Jarilo is snatched from the cradle and taken to the underworld, where Veles (god of the underworld) raises him as his own. At the spring festival, Jarilo returns from the world of the dead, bringing springtime from the ever-green underworld into the realm of the living. He meets his sister Morena and courts her. At the beginning of summer, they are married. This sacred union between brother and sister (children of the supreme god) brings fertility and abundance to earth, ensuring a bountiful harvest. And, since Jarilo was raised by Veles and his wife is the daughter of Perun, their marriage brings peace between two great gods and ensures there will be no storms to damage the harvest.
After the harvest, however, Jarilo makes a bad choice. He is unfaithful to his wife, and Morena vengefully slays him. His death returns him to the underworld. Without her husband, Morena — and all of nature with her — withers and freezes in the upcoming winter. She turns into a terrible, old, and dangerous goddess of darkness and frost, and eventually dies by the end of the year.
The whole story repeats itself anew each year. There is always a fresh springtime, followed by summer, autumn, and winter.