Challenge 020: Crossover

Our twentieth theme comes courtesy of raphiael.

Theme: Crossover
Word Count: 250-word minimum, 4000-word maximum.

You can cross over many things - the barrier between worlds, the line between life and death, the boundary between right and wrong or ally and enemy. Feel free to interpret it however you'd like. 

Entries are due by 11:59 EDT on Saturday, June 30th.  Anyone who decides they need an extension can just PM me,myaru, or starlitlady.  Happy writing!