why not a crazy devils deal thing. like lyra can trade her horn for a set of hands for three days, and if she can figure out how to fuck herself with her hands in three days she can keep them. but, if she cant, she is turned into one of these fuckers

http://vignette2.wikia.nocookie.net/disney/images/2/27/Littlemermaid-disneyscreencaps.com-2976.jpg/revision/latest?cb=20120630200249

or better yet, how about a changeling. yeah, fuck and chryssy gets to be like ursurla.

alright there are some catches. one- if she fucks herself with her hands, she has to keep the hands and lose the horn. alright, two- the hands are like t-rex hands, right. fuck that writes itself.