Pandora's box contains the 7 deadliest sins: Wrath, Gluttony, Greed, Envy, Sloth, Pride and Lust. They were all accidentally opened by the alicorn Pandora Star.
Celestia gave Pandora a box to watch over, though Pandora did not know why even though she asked Celestia. Pandora then asked her why she didn't give this box to Twilight. Celestia didn't want to put more things into Twilight's hooves. (PFFT like Celestia didn't do that already)
The seven deadly sins are: Jealous, Vanity, Laziness, Lust, Rage, Greed, and Fear. Only Hope was left in the box before it could get released. When those other sins were released, Pandora described them as 'black swirls of magic dust'.
- based on the regular Pandora's box in greek mythology
- Since hope is not a deadly sin, if it was let out, Pandora would have seen a stream of golden magical dust.