http://www.fimfiction.net/story/5481/MLP%3A-A-Favor-Returned
Eeyup. Here it is. The fun has begun, now to find a spot to make the Doc run!
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Also, for the alternate sequel,
http://www.fimfiction.net/story/5793/The-Necro-Walk
Unlike the description given at the end of this story, The Necro Walk will be following a different route: Mythica is in the process of being conquered than already having been, but as stated before he'll need to win the trust of the ponykind once again. Another thing different is that his age will NOT revert.
Whichever storyline you choose to pursue (if not both), enjoy! : )