Copying Sam completely, I’m gonna give away some PC spoiler goodness.

• All of PC is told from the point of view of a not-dead shadow king as he enacts his silent plan to get rid of the bearers of the Elements of Harmony and their successors. It starts with targeting the 3 foals that are close to the Mane 6 (the other two are just collateral). Corrupt one of them (the unicorn) and augment her magic to be powerful enough to preform dark magic that allows her to create a shadow pet thing. This pet will be the lure that’ll lead her to the shadow-world place where her power as a future bearer is tapped into and used by not-dead king to recapture his former empire. A guard would then have to send the message to Canterlot Castle. So his shadow would be corrupted and then assume the body of Princess Twilight and have her break her connection with the element of Magic (corrupt it with dark magic to be safe). Finally, the foal would be killed, breaking the chain of succession. The other elements will fall by default, and Equestria would be captured.