Helen, produced in 412, is a play in which the protagonists, Helen and Menelaus, pass from mortal danger in Egypt to escape and the promise of lasting happiness. For moderns a tragedy that ends happily is a contradiction in terms. But in Greek usage a tragôidia is a play about the figures of myth (unlike a komôidia, which is about the comic poet’s contemporaries), and any such play (apart from a satyr play, the burlesque on myth presented as the last play of a poet’s offering at the Dionysia) counts as a tragôidia. The chief requirement for a tragôidia is not that it ends in misery for the principals, but that it depicts the uncertainty of the mortal condition in relation to the gods.
Kovacs, David. "Helen". The Literary Encyclopedia. First published 13 July 2011
[http://www.litencyc.com/php/sworks.php?rec=true&UID=13361, accessed 24 October 2016.]