Sure thing, here you are!
A Song Of Ice And Fire has a consistent tradition of prologues and epilogues that follow a pattern. There are three things that are constant about every prologue/epilogue in the series: 1. They are from the perspective of a character that has never been mentioned, seen, or discussed before; a brand new character (with one exception - the epilogue of A Dance With Dragons) 2. They give us a glimpse of something "other" going on; something that is not a main plot, some machination that is lurking behind the main characters and their many problems. 3. The brand new character whose perspective we get...they die.
The epilogue of A Storm of Swords takes place a few weeks or so after the Red Wedding, and very shortly after the deaths of Joffrey, Lysa, Oberyn, and Tywin (the second half of ASOS is stupidly fast paced; by far the most action-packed book).
We are introduced to Merret Frey, riding his horse alone at night to a castle called Oldstones. The first several pages he is reflecting to himself about his bad luck recently and how much other people, even within his own House, are ragging on him all the time. He is the 9th son of Walder Frey. In passing, it is revealed that Merret is the father of Walda Frey, the fat new wife Roose Bolton got after the Red Wedding. We also find out why he is out on this trip alone tonight; he is delivering a ransom for his nephew, Petyr "Pimple" Frey, who was kidnapped by bandits. Oldstones is the meeting spot for the exchange.
Merret starts to drink. It's getting darker. He reflects on the Red Wedding, how his job was to get Greatjon Umber stinking drunk so that he wouldn't put up much of a fight. He remembers how he failed at it. He failed this, and he failed that, and the only reason his daughter got married was because she was fat and Roose was a traitor. Nothing is going Merret's way. All he needs is to get Petyr back from these bandits and maybe he will have done something right.
Merret reaches the castle. There is faint music playing. Merret locates its source, a singer sitting on a statue strumming an instrument. Merret doesn't recognize him, but the readers would; his name is Tom of Sevenstreams, and he is a member of the Brotherhood Without Banners.
Merret speaks to him briefly, says he brought the gold. Within seconds, he is surrounded. Some of the characters are described. Among them are Anguy the archer, and Thoros of Myr. There is also a figure in a cloak and hood. Merret is scared witless; he hates bandits. Thoros takes his horse by the reins and leads him into the woods...
...and show him Petyr Pimple strung up from a tree.
"But I brought the gold!"
"That was good of you, we'll see its put to good use."
"You had no right."
"We had a rope. That's right enough."
Merret is too old and weak to stop the bandits from tying a rope around his neck. He starts babbling that Walder will send an even larger ransom for him, that they shouldn't kill him.
"Lord Walder won't snap the same bait twice" says Thoros.
Merret says he'll give them any information they want. Thoros asks if Merret knows anything about the dog that ran away with the girl (The Hound and Arya) at the Red Wedding. Merret doesn't have that information. As the rope gets tighter, he asks what his crime was.
"The Red Wedding."
The rope is getting tighter. Merret struggles; he says he didn't do anything wrong during the Red Wedding. It was all Walder and Roose and the other Freys. He says he wants a trial, he says they have no proof, they have no witnesses.
"As it happens, you're wrong there." The singer turned to the hooded woman. "Milady?" The outlaws parted as she came forward, saying no word. When she lowered her hood something tightened in Merret's chest and for a moment he could not breathe. No. No, I saw her die. She was dead for a day and a night before they threw her body in the river. Raymund opened her throat from ear to ear. She was dead. Her cloak and collar hid the gash his brother's blade had made, but her face was even worse than he remembered. The flesh had gone pudding soft in the water and turned the color of curdled milk. Half her hair was gone and the rest had turned as white and brittle as a crone's. Beneath her ravaged scalp, her face was shredded skin and black blood where she had raked herself with her nails. But her eyes were the most terrible thing. Her eyes saw him, and they hated. "She don't speak," said the big man in the yellow cloak. "You bloody bastards cut her throat too deep for that. But she remembers." He turned to the dead woman and said, "What do you say, m'lady? Was he part of it?" Lady Catelyn's eyes never left him. She nodded. Merret Frey opened his mouth to plead, but the noose choked off his words. His feet left the ground, the rope cutting deep into the soft flesh beneath his chin. Up into the air he jerked, kicking and twisting, up and up and up.
That's the epilogue, and that's the first we see of Lady Stoneheart, AKA Catelyn Stark risen from the dead to lynch Freys.
"Stoneheart" is a name we only know she has much later, towards the end of A Feast For Crows when she appears again. It's also explained how she is alive; her body was pulled from the river (by Nymeria! Arya's wolf!) and was found by Beric Dondarrion, leader of the Brotherhood Without Banners. Thoros tried to revive her as he did with Beric numerous times, but couldn't. Beric then performed a heretofore unexplained magical act where he transferred his own life force to Catelyn. Now she is alive, he is dead (for good), and she now leads the BWB and is running amok in the Riverlands, stringing up Freys.
I'm not going to go in to why this is important, because that is straying into possible show-spoiler territory. But from this point on, the fact that there is a hooded woman running around in the Riverlands becomes common knowledge. It tangentially affects a lot of plot points, and some of the more dedicated hardcore readers have pieced together the various hints to figure out what exactly is going on in this shadowy subplot that we see so little of. It comes to a cliffhanger at the end of A Feast For Crows/A Dance With Dragons, and we've been there ever since.
Needless to say, if this had been the closing scene of Season 4 of the TV show, it would've likely been an even bigger television phenomenon than the Red Wedding.