Hm… A quick summary? Sorry, quick and short summaries, have never really been my strong suit. So, take a seat, have a cookie and listen… ^^
Well, big surprise, he’s still alive. He’s gotten old, though. He lives with some animals in a cave below the throne room in Canterlot. He invites the Mane 5 to what he calls a little soiree (tea party). His magic also seems to glitch, by the way.
Anyway, he tells how he fought the good fight with the Mane 6; and of course exaggerates his own part in it. But he says: “Twilight made sure everypony in Equestria felt respected and cared for, and all lived in harmony.” And how he and Fluttershy lived together in the animal sanctuary. “Until a wicked pony came along, one who caused pandemonium with her false ideas of a perfect Equestria.”
You can only see this pony as a shadow figure, but she literally says, “Unicorns have better things to do than wait in line.” She also knocks Fluttershy to the ground. After that, you see how everyone in Ponyville starts to distrust each other. Also, Discord and Fluttershy gradually lose their students because their parents no longer allow them to associate with them, which makes the Lord of Chaos weep inconsolably in the flashback. Although he tells the Mane 5 that of course he had to hold it together for Fluttershy.
And Discord says, “Then truly terrible things happened, much of which is far too frightful to describe.” He and Fluttershy are seen fleeing from storm clouds floating overhead. Then you see the image of the Mane 6 creating the Unity Crystals, and how peace returns to Ponyville.
Asking if a society without magic might not be more peaceful, he reveals to Sunny that he is the crystal thief. When the crystals were reunited and magic returned, he would have panicked, fearing that the ponies might use magic against each other again.
Of course, the Mane 5 get mad about this and want the stolen crystal back. When Zipp asks what he plans to do with it, he says that’s why he lured them to his cave. He does not know. This decision would be too big for him alone. But because Sunny, Hitch, Zipp & Pipp want to embrace magic, he turns to Izzy, who reminds him of Fluttershy. She was previously left alone in the forest.
And although Izzy don’t agree with him - she actually hesitates to give a clear answer - he says “But I’m going fix this for you, Izyy”. That he would find a way to destroy the crystal. He uses his chaos magic to tie the Mane 5 to the ground. Sunny thinks Twilight’s power would be too much for him to break the on protection spell from her. But he is sure that with a little reseach he will find a way. And with the words “I hope we reunite for another tea party during more peaceful times” it teleports away. This is more or less the end of the comic. To be continued…
Short enough? ^^
Edit: Perhaps briefly stated the reason he wants to fix it for Izzy. Fluttershy would have always made excuses for others throughout the years, but he could have seen how much it had hurt her. Just as he sees in Izzy how hurt she is about being left alone. He also says, “It’s hard to feel left behind simply because of magic you don’t possess.“
My personal opinion: Whether you like Discord or not. He certainly has an important point. Whether that is reason enough to destroy the crystal in his hands, well, that would have to be discussed. It is in any case a typical Discord move. ;-)