Baby

Dean Winchester loves four things: his brother, Cas, pie, and Baby. Anyone who messes with any of those four things are sure to get Hell from Dean.
But what happens when one of those things change?
After a hunt for witches, Dean notices something wrong with Baby, but he's too tired to take care of it. What he doesn't expect his that his Baby will change everyday until he finds out just what exactly that sigil is and how to break the curse.