The Promised Day goes very, very wrong. Edward Elric, thrust through to the other side of the Gate, grows a deep distrust of wizards and magic, when Nicholas Flamel sends his Philosopher's Stone to Hogwarts, Ed takes it upon himself to keep it safe from them, even if he has to face his past mistakes. Crossposted on Ao3
The Philosopher's Stone was never meant to come into contact with mortal blood. What changes will be wrought? Just remember Equivalent Exchange is the name of the game. Will go through HP universe leading into the FMA universe (summary may change)