Common key themes in the legends of demigods in mythology include heroism, identity, and the struggle between divine and mortal realms. Demigods often face challenges that test their strength, wisdom, and morality, serving as symbols of the human condition. Many narratives explore the concept of destiny, where demigods grapple with their heritage while striving to forge their paths. Themes of sacrifice and the quest for belonging also permeate these legends, highlighting the ongoing influence of their divine ancestry.