You sure you don't already know what's going to happen?
"One week," Twister thought. It was just one week ago Starswirl expelled him as his pupil because his magic was "too chaotic and unpredictable". Now today Starswirl was dead. Twister wished he could say he was sad. He respected Starswirl, obviously, but he had only known him for two weeks before he was expelled. He still remembered the little incident with the sun raising spell. Some of the ponies still had the sunburn to prove it happened. He really couldn't blame Starswirl for thinking his magic was chaotic, but he felt expelling him wasn't really the smartest thing to do. And he's supposed to be the all-knowing, wise wizard!
He looked out of his window and sighed. Equestria's citizens had finally settled down. There were not many ponies coming in to live now. All the hustle and bustle of building another nation had finally died down, and frankly... it was kind of boring. He missed all the ponies coming in, all the noise and chatter outside, all the construction workers working... actually, no. He was glad the construction workers were done, in his area at least. He still remembered when Clover the Clever returned and told the unicorns that they had found new land! And something about friendship and harmony with the other tribes, but that wasn't as important to him.
Actually, speaking of which, if the tribes had become friends, why were there mostly unicorns in his area? There were only a few earth ponies, and he had never even seen a pegasus since they moved to Equestria. Then again, that was probably because he didn't talk to other ponies much. Not that he wanted or needed to.
Everything was so boring he decided it would be better to go to sleep. When he entered his messy room, he tripped over something. It was his old notebook from when he was Starswirl's student. He opened it and came across one of those spells that the bearded wizard never finished. He never exactly told him what it did, and the wording was too vague to tell anything. Still, it didn't look too dangerous, and Twister really needed something exciting to pass the time with. He cast the spell.
"From one to another, another to one. A mark of one's destiny singled out alone, fulfilled."
For few moments, nothing seemed to happen. Disappointed, he dropped the book back down on the floor. "Over two hundred spells," he thought. "How many of them actually do something?"
He got his answer. All of a sudden a very bright light blinded his eyes, and he felt his body changing. His horn became zigzagged and he sprouted an antler. He also grew wings and his front hooves transformed into a paw and a claw. When the strange magic finally subsided, Twister's first question was, "Why is everything so short all of a sudden?"
When he looked in the mirror, he screamed and a lot of other questions came to mind. "Why do I look like this? I'm a mishmash of every animal known to ponykind! Why do I sound like this? Why do I feel so powerful?" After calming down a bit, he asked himself the third question again, this time with curiosity. "Why do I feel so powerful?"
Just to experiment, he tried to perform a spell he had never been able to do: change form. He thought of something weird, like a hybrid of all the pony tribes (like something like that could ever exist!). It worked with great ease! If he could do that, he could surely do anything he wanted! Like make things around Equestria a bit more interesting for starters...
A few cotton candy clouds raining down chocolate milk was all it took. Twister wasn't himself anymore. He started calling himself Discord and doing stuff that fit his new name quite well. He made fish "swim" on land, made ponies sneeze loud enough to collapse (fake) buildings, and made the sun and moon rise and set every few minutes, among other things.
Over time, his tricks got more and more dangerous. He took away unicorn horns and pegasus wings, sometimes replacing them with other things like ice cream cones or insect wings, he turned houses upside down, and his personal favorite: played mind games with ponies and forced them to act the opposite of what they usually would. That last one was reserved for the annoying goody-goodies who had a lot of friends. He even took over Equestria (which wasn't that hard, considering there was no specific pony ruling it). There was a minor run in with two ponies who apparently had horns and wings, but he took care of them.
Discord conjured up a throne and smiled down at all the beautiful havoc and disorder he had caused. It was hard to believe that just weeks (or months? He couldn't remember, neither did he care) ago, he was merely a unicorn with just a bit of chaotic magic. Now he was the spirit of chaos and ruler of Equestria! He sipped a glass of chocolate milk. The sight of watching ponies run around in confusion was just icing on the exploding cake. Every few minutes or so, he snapped his fingers and changed the time of the day, sometimes even the weather if he felt really good, which was quite often.
Just then, he overheard the two tall hybrid ponies talking about something called the Tree of Harmony. It sounded like a potential obstacle for him and his glorious reign of disorder. Worse, those mares might take over and make everything peaceful! Discord cringed at the thought.
"We can't have that, now can we?" Discord thought as he conjured up a bag of black seeds, grinning deviously.
The rest, as they say, is history.