I believe the Persian ruler in question is King Khuzroi, who lived during the 7th century. Khuzroi, who is described in various sources as "mad" and "cruel," originally intended to wipe out the entire family of the Jewish royal house of David. Most of the family is either killed or imprisoned. However, in a dream, Khuzroi finds himself in a beautiful garden. When the king tried to kill a tree with an axe, an old man prevented him from doing so. The old man took the axe and prepared to kill the king. The old man agreed to allow the king to live if the king promised to preserve the tree that he had nearly chopped down. The king called forth one of his Jewish prisoners, who interpreted the dream; symbolically, the tree signified the House of David. The king then decided to seek out the pregant mother of Bustenai (meaning "garden"). The king gave special treatment to Bustenai, who he eventually named "Leader of the Exile."