Sombritis

Doctor Whooves had been up all night studying this disease. With his trusty assistant Derpy by his side, he was in the process of formulating a cure when he heard on the news that he and Derpy were the only survivors.

"Well...", said Whooves nervously. "That's not good now, is it?".

"Yeah!", said Derpy, who was quite scared at the moment.

"Hmm.", inquired the doctor. "It seems like the disease was created by another being of this world. That particular being was King Sombra, although he was seemingly banished to the far north, north of the Crystal Empire. Now, he has returned.".

"But how?", asked Derpy. "Sombra WAS banished forever, wasn't he?".

"It WOULD seem that way.", said the doctor. "But nothing lasts forever in this world, which means we MUST find a cure before the whole of Equestria succumbs to this awful infection. And while the news said there are no survivors except the two of us, they don't know that all the way... yet."