Looks at first like Sombra simply attacking Twilight with Dark Magic, and then you notice her eyes. Things aren't quite so one-sided. ^_-A picture commissioned for a scene from 'A Somber Tale', and drawn by the talented. Here is the scene, for those that are interested.

Sombra’s voice whispered in her ear, “You can only imagine my shock when I heard that particular piece of information. Strange how you failed to mention that your current teacher was one of my worst enemies, is it not?”

Twilight immediately jumped away from the stallion. She would have expected annoyance, even anger, but the cold emotionless tone with which he spoke caused an icy trickle of fear to run down her spine. His expression was just as neutral, giving no hints as to what he was thinking. “I wasn’t trying to hide it,” Twilight protested, “it just never came up.”

“Oh, do not misunderstand me.” Sombra said, still unnervingly calm. “I can appreciate why you would keep it from me. You are no fool, and must have been worried that I wouldn’t teach you if I knew. However, knowing would have saved me some time, since Celestia would no doubt ensure that you have a satisfactory knowledge of geography, history, mathematics, basic and advanced Unicorn Magic theory, and many other subjects.”

“Then… you’re not angry?” Twilight asked hopefully.

The silence that stretched out before he responded was very telling. “…I believe it is time to start your lesson.” As soon as that was said, Sombra calm veneer shattered, his Dark Magic forming an aura more powerful than Twilight had ever seen.

She barely managed to dodge the attack that immediately followed.

“Tonight, we will be focusing on strategy, tactics, the difference between the two, and your reaction time,” Sombra continued to speak as if he was not currently trying to kill his student, while simultaneously launching one attack after another. “Since this is a dream, even an attack that would be fatal in reality will merely wake you up, here.” A feral grin spread across Sombra’s face. “You pass the lesson if you manage to get a good night’s sleep.”

Needless to say, Twilight was quite tired indeed, when Celestia rose the sun that morning.