Xeethra

"Xeethra" is the tale of the orphaned goatherd Xeethra, who tolerates his abusive uncle and lives in dire poverty. One day, he spots a cave in a hidden valley, and descends to a beautiful but sinister garden, where he eats a weird fruit hanging from a tree. He starts having memories of being Amero, the ruler of a far-off country, and when he returns to the surface he sets off to seek his kingdom....

This short story is the first in the collection ZOTHIQUE, which was part of Ballantine's Adult Fantasy line, coming out in 1970. The story itself was first published in "Weird Tales", December 1934.