Invisible Like a Ghost

Sir Perseus
in your cap you did quite nonpluss
poor Medusa sleeping in her cave.
Pure trickery that forced her to an early grave.

(napowrimo day 14….I tried to combine 3 prompts today: http://www.writersdigest.com/whats-new/2017-april-pad-challenge-day-14 , http://withrealtoads.blogspot.com/2017/04/invisible.html , and here: http://wintercearig.com/2017/04/14/2017-april-pad-challenge-day-14/ )