"For the anniversary of my reformation, I present to you four a gift: A Golden Apple. Not just any golden apple, mind you; this apple is very special, for it can go only - it says here - "To the Cleverest" of them all (that doesn't mean being an egghead, Dusk Shine). Now who would that be among you lot?"
--- Eris, offering the Golden Apple to the assembled Alicorn Princes Solaris, Artemis, Bolero and Dusk Shine.
True to her expectations, the Alicorn Princes interpreted cleverest as 'who can one-up the other' the most. The resulting prank war wrecked the Canterlot and Crystal palaces, devastated the local areas and even defaced the Moon.
Eris: "And the Four Naive Stallions follow it but I admit wasn't part of the destroying the cities, dear Bolero~ Any of you ponies could stop at this point, but there's the result is...that no pony did. I'm sure anyone of you were still conscient on your actions...~ Aside that I won't let the true Golden Apple get in simple hooves"