There are two stories about the god or Gods of this name. As the child god whom Aphrodite loves he is redolent of Eros, and is gored to death by a boar (at the behest of the Goddess) while hunting; it is this story that William Shakespeare tells in his Venus and Adonis (1593). The other (or continuing) story has perhaps more resonance within modern fantasy: as the god whom Persephone loves, Adonis is directed by Zeus to spend half (or a third of) the year above-ground and the remainder in Hades. In this aspect he is one of several gods of vegetation, and Rituals dedicated to Adonis – they occur at the Season of the harvest – annually mourn his death, which is the death of the consort of the Goddess. His story is very close to that of the Mesopotamian god Tammuz (> Mesopotamian Epic).
In From Ritual to Romance (1920) Jessie L Weston identifies Adonis with the Fisher King, whose Healing through the finding of the Grail represents (in her view) the seasonal rebirth of the land. Weston's ideas are no longer taken very seriously by scholars. [JC]