When King Sombra was destroyed, his horn remained intact after the explosion. What would happen if someone found that horn and used it to bring back King Sombra?

NOTE:

Blue_Moon is the author of the prologue and first three chapters, he handed it over to me a little bit ago.

Everything seemed to be alright the day that Discord came back to Canterlot under the watchful eye of Celestia as a (mostly) redeemed draconequus. But the luxurious life of royalty is so... BORING! "Do something creative," they said, "It'll be fun," they said. Thus it was that Discord sat down and began writing the worst stories he could imagine! But between posh living, running a ministry, court dates, random quests from Celestia and Luna, and learning about the magic of friendship, surely these stories can't be that bad, can they?