So many close calls, so many potential hesitations, so many things that Twilight and Spike could have done differently, and they'd have failed. It doesn't matter what they did wrong or didn't do right, only that a split-second is all the difference Sombra needed. Now, Twilight is completely at Sombra's mercy, subject to whatever mind games he has planned for her...
Recent viewing of The Crystal Empire is helpful but hopefully not necessary. Special thanks to Mickeymonster for drawing the image that inspired this story.