In ancient Greek literature, an eidolon (plural: eidola) (Greek εἴδωλον: "image, idol, double, apparition, phantom, ghost") 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 which 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 War, claim that Helen was never physically present in the city at all.
The concept of the eidolons of the dead was explored in various literature regarding Penelope, who in later works was constantly laboring against the eidolons of Clytamnestra and later Helen herself.
Modern World 
- In The Mark of Athena by Rick Riordan, three eidolons appear as antagonists. They possess Leo Valdez into starting a war among Romans and Greeks. Then they possess Percy Jackson and Jason Grace into trying to kill each other. At the novel's climax, the three eidolons try to kill Valdez, Frank Zhang, and Hazel Levesque. Their former victim, Valdez, saves Zhang and Levesque by defeating the eidolons, through his mechanical knowledge and the help of the goddess Nemesis.
See also 
- Holmberg, Ingrid E. (Spring, 1995). "Euripides' Helen: Most Noble and Most Chaste". The American Journal of Philology (The Johns Hopkins University Press) 116 (1): 19–42. JSTOR 295501.
- Meltzer, Gary S. (Oct. 1994). "Where Is the Glory of Troy?" "Kleos" in Euripides' "Helen" 13 (2). University of California Press. pp. 234–255. JSTOR 25011015.
- Papi, Donatella Galeotti (1987). "Victors and Sufferers in Euripides' Helen". The American Journal of Philology (The Johns Hopkins University Press) 108 (1): 27–40. JSTOR 294912.
- Barasch, Moshe (2005). "The Departing Soul. The Long Life of a Medieval Creation". Artibus et Historiae (IRSA s.c.) 26 (52): 13–28. JSTOR 20067095.
- Carpenter, Frederic I. (Mar., 1942). "Walt Whitman's "Eidolon"". College English (National Council of Teachers of English) 3 (6): 534–545. JSTOR 370944.
- http://www.theosociety.org/pasadena/etgloss/etg-hp.htm ENCYCLOPEDIC THEOSOPHICAL GLOSSARY: A Resource on Theosophy, G. de Purucker