In ancient Greek literature, an eidolon is a spirit-image of a living or dead person; a shade or phantom look-alike of the human form. The concept of Helen of Troy's eidolon was explored both by Homer and Euripides. However, where Homer uses the concept as a free-standing idea that gives Helen life after death, Euripides entangles it with the idea of kleos, one being the product of the other. Both Euripides and Stesichorus, in their respective works concerning the Trojan Horse, claim that Helen was never physically present in the city at all. (source)