A human is sent to Equestria by Princess Luna, whom he meets in a forest. He meets the Mane 6 but is not trusted, as he is a foreigner to their land. He accidentally is transformed into a pony(Without a Cutie Mark, joining the CMCs possible?) by Princess Luna's "drinking with an old friend" & discovers that he is the one chosen for special powers. He even falls in love with a pony.
But he accidentally ensues an event that brings chaos into Equestria that hasn't been seen for 1000 years. Some friendlies and villians not even from Equestria. The human is charged to fix what he set in motion, with some help, to protect Equestria from certain doom, & stop the mastermind behind it all.

"The Kobald must unite the earth to fight alongside the Five,
Or else the Butcher will rule with the mirror at his side."
-The One