Updating this because people have brought up some REALLY GREAT plot-holes and I like trying to flesh out my AUs soooooooo…
Ginny is the one who suggests using the Chamber. Of course she is. Harry isn’t the type to think of that, but for Ginny… for Ginny the Chamber still haunts her dreams, too often, and she’s furious that a part of the castle is restricted to her - a part of her home that she wants to avoid. She suggests the Chamber, partly for the DA’s benefit, mostly for her own.
Visibility is a concern - what if someone sees them going into the girl’s restroom? They think it’s a serious issue until Ron starts laughing. No one comes near that bathroom anymore, he says. Not ever. It was barely an issue while brewing a month long polyjuice potion, Ron and Harry popping in and out to add ingredients or to stir. Now though? Now that Myrtle has stepped up her game (shrieking, flooding the room if someone unwanted comes near), now that Hogwarts is infused with rumors that Harry fought a basilisk right in there, now that the nearby corridor still has a bloody, horrifying message that even the professors haven’t been able to erase*… well, students avoid the area like the plague.
Even if they didn’t, the House Elves help them out. Dobby did, after all, suggest the Room of Requirement before Ginny brought up the Chamber. Who better than the workers who see but are not seen to help the DA keep watch?
The castle helps too. By now it knows Harry and desperately wants to protect its students. More than once Umbridge follows a DA member, only to find the staircase moving unexpectedly, taking her in another direction entirely. Sometimes there’s even a door directly beside the lavatory - appearing out of nowhere - that students can slip inside if they feel the need…
Getting out is the other concern. At first they think to bring brooms or levitate one another out… but that’s just not practical. Then, one of the Hufflepuffs asks the obvious and yet oddly illusive question: how did Salazar get out? They start a search and by the end of the day they’ve found at least four hidden exits.
One exit leads out into the Forbidden Forest, a space that’s not nearly as terrifying as it once was. Harry speaks quietly to Firenze and secures the help of the centaurs for when they need safe passage late at night. One day they encounter a group of acromantulas… and Harry learns of Hagrid’s strict new rule - friends of Hagrid are never food, no matter how easy the prey. The students don’t realize it, but they’re slowly gaining allies. Those in the forest begin to take notice of the children who walk both bravely and respectfully through their trees.
(And one day when they’re too tired to walk back, a familiar blue car pulls up and throws open its doors. Ron cheers like a maniac. Ginny laughs and threatens to tell their dad).
Though the exists are great, it’s Hermione who realizes the Chamber’s true benefit - it lies outside of Hogwart’s apparition zone. How can it not? Godric, Helga, and Rowena didn’t know of its existence when they first made the wards. So now the DA can go with ease, they just can’t pop in from anywhere else in the castle. Which is, admittedly, perfect. Apparition lessons begin in earnest.
(And during the Battle of Hogwarts, DA members take Slytherin students by the hand - those who wouldn’t, couldn’t, fight their own families. They take them down to the Chamber and tell them to apparate out. Leave while you still can. Keep safe).
Harry realizing that parseltongue is easily imitated and coming up with an actual password that has to be spoken, one linked to a spell too. It helps that the snakes around the entrance are semi-sentient and are loyal to their new master. They know who’s meant to go down there and who’s not.
Neville joking one day that they should be learning how to use swords, considering that’s how the original battle down here was won. Harry takes it seriously. Not the swords bit, but using physical/muggle fighting techniques on wizards who are too reliant on their magic. They begin reading up on hand-to-hand combat and knives.
Harry needing to test their progress and getting a really stupid idea… but honestly, those often work out in his favor. So one sunny, Saturday morning - when everyone else is lounging outside - Harry sneaks the DA into the third floor corridor. Fluffy is gone, as is the mirror, but the rest remains, no doubt left in case Dumbledore ever had to guard something else precious. Hermione, Ron, and Harry spend the day supervising, teaching their peers how to react under pressure, think through situations, and rely on one another’s skills.
And then one day things get weird (because they always do with Harry) when he realizes that the miniature chamber the basilisk was kept in is the only part of their hideout they’d yet to explore. See, given their rarity, it’s unsurprising that wizardkind knows so little about basilisks - not that they reproduce asexually or that only a parseltongue can hatch the egg. So when Harry crawls into the chamber, and finds a strange egg-like object nestled there, that begins pulsing a soft green color in his presence, and when he basically says, “What the hell…?” out loud, and when it comes out in parseltongue because he is surrounded by snake things…well, let’s just say a few minutes later Harry crawls back out, very sheepish, a baby basilisk cooing around his neck. He laughs pretty shakily and mutters something about finding their mascot.
(And they name the beast - because of course they do - and Hermione invents a soft device to cover its eyes and feeding it is an absolute horror… but they do grow to love their ‘mascot.’ And during the Battle - when Harry is off in the forest and Hogwarts is losing badly - no one is more surprised than the Death Eaters when Ron and Hermione come tearing out of the school riding a goddamn fully grown basilisk. Hermione rips off the cover on its eyes and sets to work).
* “In the book, it says that Filch could not get the messages written by Ginny off of the wall. It is unknown if he ever did, and it has not been mentioned since.”
