Discord could roam the castle of Canterlot to his heart(if he had one, of course)'s content, but instead he chose to stay secluded in his chambers whenever possible. Why? Because he was experimenting. Experimenting with possibilities that up until that moment had never even been considered. At first he had been desperate for revenge against the elements who had trapped him in stone, but when he was released he encountered a devious mind even more clever than his own. He introduced his plan to her, and she gladly accepted.
His experiments still had everything to do with revenge, but in an evil and twisted way that would enslave the elements to his will. All but the one who had assisted him. So he need a replacement, and chose a pegasus with powers beyond her own knowledge, powers that could rival even the great Princess Celestia. Discord had finally perfected his dark magic. Dark magic to create Dark Ponies.