The song is from season 6 episode 8 of MLP:FiM.
 ♫ Lyrics for the song: 
( You can support the show here: https://itunes.apple.com/us/tv-season/my-little-pony-friendship/id1081667436 )
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[Princess Luna as the Spirit of Hearth's Warming Yet to Come]

I see a cold wind blowing through 
I see days neither fun nor free 
I see a future caused by you 
I see a path not meant to be 

The future should be filled with magic 
Dreams and wishes brought to life 
But the days ahead are dark and tragic 
No time for hope when all is strife 

Whatever might have been 
All the dreams that ponies share 
Because of you, Snowfall Frost 
Now the future is a cold nightmare 

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"My Little Pony" is owned by Hasbro, 
"MLP: Friendship is Magic" is created by Lauren Faust.