Darkness is a living thing. It consumes the strong and destroys the weak, not knowing the meaning of mercy. It corrupts and devours until there is no light left within one's heart.
What happens when the young mare who wields the Element of Magic succumbs to the Darkness?
Spike never went down the stairway to save Twilight from Sombra's door. This is what happened after.
Twilight has done things she regrets. Things that she wishes she could take back. But nothing she can do will ever fix the past. And when the forces of Darkness at work again, she fears she may only have more regrets.