She has been seduced by the forbidden knowledge of the great old ones! Her scrawling only seem chaotic to us because we cannot see the other halves of the writing contained in the broken dimensions she has been granted access to!
The light in her eyes, the vibrancy of the text; they are infused with the powers of the void! The end is nigh!
I think that she's waking up after just being teleported, via that circle, outside of Equestria For a speed paint it's pretty good, the dazed look on Twilight works well in the pink light, as does the contrast between the natural and magical light Only criticism, very minor but my perfectionist side won't ignore it, is that her cutie mark here is a 8-pointed start instead of 6-pointed
So much was sacrificed to get Twilight there. So many have died. The villains gloated that they had won, that Equestria had fallen. They cast her aside in that cave - defeated, but they didn't count on her determination. With the last of her strength she draws the runes on the floor and casts the spell. It takes everything she has left. Darkness comes for her, but she knows: Equestria is safe again.
-> Please don't take that as an attempt at writing. It's just as close as I can get to describing what I see in that image...