Summary: There are three secrets in the Slytherin Common Room: the rug, the footstool and the List.
Disclaimer: the Harry Potter universe et al belongs to JK Rowling.
The List
Hogwarts is full of secrets and even Albus Dumbledore never claimed to know them all. To first years the locations of the entrances to the other Houses are secret and the missing steps on certain staircases come as a surprise to those not 'in the know'. Later they graduate to secret passageways, how to get into the kitchens and passwords for the Prefects Bathroom and other hidden places. Later still it's the places Filch sometimes forgets to check after curfew and where Trelawney hides her stash of sherry.
There are three secrets in the Slytherin Common Room: the rug, the footstool and the List. Students seem to forget that their Head of House was once a student, so the secrets don't get talked about much for fear he might find out.
The rug is an enormous green and silver masterpiece, one of many that cover the cold stone floor and make the room look inviting. However, this one is believed to have been commissioned by Salazar himself. Two life-size snakes have been painstakingly embroidered in the shape of the initials SS in one corner. And it is said to eat Muggleborns.
This current generation of students have never seen the rug in action, but then having a mudblood inside their common room would be unthinkable, even if it was to test out the rumours. Still, a Mudblood Eating Rug is something to be proud of.
Unbeknown to his students Professor Snape knew all too well about the footstool. Once upon a time and not so very long ago there was a young transfiguration teacher called Professor McGonagall who set the dubious task of turning inanimate objects into living objects for homework, specifically small items of furniture into small mammals, since no one had been able to manage it within the actual class. Determined to show their ease with using the magic their owned by birthright (a.k.a. desperate not to be shown up by some muggleborn Ravenclaw in the next lesson) the Slytherins formed an emergency study group and spent all week trying to master the spell. Eventually, in a fit of temper, Snape had turned an innocent footstool tartan before blasting it to smithereens.
"Impressive," a charismatic older student had laughed before repairing it and tutoring him for a whole five minutes in the complicated wand gesture required for the correct transfiguration. After that the spell had been easy and they'd turned the footstool into a hedgehog, a rat, a rather ugly toad and a tabby cat; and Severus Snape had idolized Lucius Malfoy from then on.
The footstool is still tartan and meows when kicked. (And, from time to time, Snape has been tempted to make another one as a Christmas present for Minerva).
However the List – which is fully deserving of the capital letter - Snape knew nothing about. It's a fairly recent addition to the Slytherin Common Room, spelled so that no one who isn't a student in the House can see it. It's not that anyone is embarrassed about the fact that it's there, but it's just a piece of parchment and it would be very easy for the someone to get rid of it if they wanted to, or if the House elves thought it was a bit of rubbish and pulled it down, or something.
It's just a piece of parchment attached to the wall above the main fireplace with a sticking charm so that everyone who knows it's there can see it. It doesn't look like anything important, just a list of names written in black ink in all different styles of handwriting.
The first name is Terence (Terry) Higgs. The rest of the school remembered Higgs being a fairly average Seeker on the Slytherin House Quidditch Team, if they remembered him at all. Miles Bletchley and the rest of his roommates remembered a small boy with a continuously running nose who grew into a skinny young man with a large pocket handkerchief that they used to hide after quidditch practise.
The newspapers remembered him for a day as a halfblood with the dubious honour of being a member of one of the first families to be murdered in the Second War. Unofficially there had been others, deliberately ignored by the ministry, but it was hard for them to ignore the Dark Mark floating over the house of a family who were close friends of the Montagues.
The List didn't exist when Terry died, but when it was begun no one said anything as Miles carefully and neatly scribed his name at the very top.
The second name is almost carved into the parchment, as if written by a child determined to get the job done right. This is rather apt as it practically describes Richard Bole's life. He tried, and failed mostly, at everything he was ever asked to do. His habit of acting before thinking, if he thought at all, made him a great Beater on the House team, as far as his Captain was concerned, and a very obedient son.
So obedient that when his father ordered him to fight anyone who approached their house, whilst he and his wife hastily packed their most important possessions for a sudden trip to Switzerland, Richard shot off two curses and a hex at the five Aurors who apparated onto the front lawn. All three went to Azkaban and Richard received a Dementor's Kiss.
Marcus Flint scrawled Richard (Best Beater Ever) Bole, polished his wand with a grim smile and set off for the infamous wizarding prison where he slaughtered three of the Aurors standing guard by the boat used to ferry prisoners across to the Isle before becoming the third name on the List.
The fourth, Graham Pritchard, shouldn't have been written there. He shouldn't have been home that weekend. The Hogwarts' hospital wing was perfectly capable of looking after a student with the measles, contagious though they may be, but Mrs Pritchard was rather old fashioned and Mr Pritchard had made valuable donations to the school in the past, so Graham went home just in time to see a bunch of Death Eaters interrogate his father.
There was too much blood on the floor for it to have been a simple murder.
Malcolm's hand was shaking too much to put his best friend's name on paper, so Blaise had gently taken the quill off him.
The Christmas holidays came and then Albert Derrick (another quidditch player), Suzanne Parkinson (had curled her hair for the last Yule Ball) and Persephone (Sef) Critchley (who skipped breakfast at lot because she hated waking up). Easter fled, another summer loomed and students left the Great Hall clutching the 'Daily Prophet' and muttering "Death-Eating Slytherins".
Greg curled up on one of the soft couches and Millie sat down next to him, almost touching. Daphne looked at nobody, Blaise's arm slung around her shoulders, and Theo stared at the floor.
Draco Malfoy who took the mark because he wanted his mother to live, and did as his father told him and then didn't; Vincent Crabbe who followed him because that's what friends do; Severus Snape who stood up for the students in his House regardless; outspoken Pansy Parkinson who spat in her brother's eye as he cursed her; Adrian Pucey,whose young squib sister was murdered by stupid Muggles and one of their stupid driving machines; quiet, sensible Tracey Davis in the wrong place at the wrong time.
Outside of Slytherin House there were Death Eaters and Victims and Evil. If there was a commemorative plaque when the war finished, whichever side won, not all the names on the List would be recalled. Inside of Slytherin House there was the List. If you loved someone you could still hate them at the same time, 'Death Eater' was not synonymous with 'evil' and everyone deserved to be mourned.
The seventh year Slytherin students didn't see much point in leaving their common room. Theo wondered if anyone would notice if they just stayed there when school finished and never left.
Maybe they could be the fourth secret.