You need the following items for your Fiona from Shrek Halloween costume:
Fiona Halloween Costume
Fiona was born in the kingdom of Far Far Away as the only child of King Harold and queen Lillian. At some point in her early years, she was secretely bethrothed to Prince Charming by her father as repayment to the Fairy Godmother for turning him human. This marriage would never take place however.
Fiona was under a curse (possibly casted by the fairy godmother) that between sunrise and sunset she was a human, and at twilight an ogre.