Discord never betrayed Equestria for Tirek, aiding in returning the magic to all the drained unicorns. As a result, Celestia deems him ready of a "small" gift: The Archduchy of the Everfree, technically making him the highest noble under the princesses.

Naturally, this shocks everypony, including Discord himself. Now of course his lands will some have some chaotic magic and strange laws in effect, but that doesn't mean he doesn't take the noble thing a little seriously. Why is anypony's guess, but it'll be entertaining to watch.