Help me I have to write a story in english and I need help plz!?

okay so I have to write a paper in english and it has to be about a troll as the character/hero, the setting has to be a underground cavern an important objet has to be a charm or a magical saying ex":( hocus pocus)" and the theme has to be about descent into the underworld and to come back from it. Please help I dont get it or what it should be about how I should write it and what should happen PLEASE HELP!