Finished reading Volume 16 last night so I thought I'd jot down a basic summary and information that has been revealed. Spoilers for those of you following the translation on BT!
Summary - Chapters 1-4
At the end of 15, we saw the birth of the Calamity Armor Mk-II. It was created using Wolfram Cerberus's third personality (Noumi/Dusk Taker) to steal away Scarlet Rain's Enhanced Armaments, in addition to the ISS Kit main body's negative energy being transferred to it. However, they didn't manage to steal ALL of Scarlet Rain's "Invincible". "Invincible" is made up of four parts: the cockpit block, around which the other parts are mounted; the thrusters on the back; the main cannon; and the missile pods. As Yuniko says later on in the book, she can still summon Invincible as long as the cockpit block is there. Unfortunately for the Acceleration Research Society, Wolfram Cerberus III/Dusk Taker stole everything but the missile pods.
In addition, it appears that the defeat of the ISS Kit happened far too early for the ARS's plans. It hadn't gained enough energy for their liking yet, and not all of Scarlet Rain's armaments were secured.
The group in Tokyo Midtown Tower - Crow, Pile, Bell, Rain, and Leopard - face off against the Calamity Armor Mk-II, fueled by all the dark energy gathered by the ISS Kit's main body via the other Burst Linkers. Crow plans to have everyone dodge when it fires the main cannon - as it's the Armor's only real weapon at this point - because they should have the reflexes to see it coming as it's charging up.
However, as it's about to fire, Crow gets an unconscious warning signal via Metatron, and instead he carries the other four Avatars and flies upwards as fast as possible. When the Armor fires the cannon, it's powerful enough to pretty much level the entire school area (even though, again, the terrain in the courtyard is supposed to be indestructable), and all of Tokyo Midtown Tower along with it. There's now a giant crater where the tower used to be, and of course, the Armor is unharmed Leopard immediately starts keeping track of how long the cannon will take to fire again.
Meanwhile, the ISS Kit's main body is melting into the pit of lava created by Ardor Maiden's fourth-quadrant Incarnate attack. It seems like it's dead, but suddenly a small ball comes out of it and assails the heavily damaged Black Lotus with tentacles. They're able to fend it off because the ghost of Red Rider kept his promise and deactivated all the ISS Kits as the main body was dying.
Speaking of which, there was a good chance that the ISS Kit's main body was actually a Duel Avatar. By destroying it, they should get its name from its death marker... but to everyone's surprise, the eyeball actually dissipates into light and disappears into the air: it's the permadeath effect when a Duel Avatar has been drained of all its points. The portal appears underneath it afterwards.
However, they decide not to follow through with their original plan to decelerate and pull the plug on Yuniko's cable, since they realize that something has happened (the red laser thing came out of the ISS Kit body and headed straight for the other group's location) and they had seen the Armor's first shot. They immediately begin to head in that direction, but they're pretty far away and pretty beat up already.
Meanwhile, the other group, still in the air clinging to Crow, see the Armor about to fire at them for a second time. Leopard notes that it was exactly 60 seconds of charge-up time. When it fires at them, Crow dives underneath the blast with the help of the Metatron Wings and everyone goes straight for its presumed weak spot: the single glowing eye in its head. The first attacks to hit are the ranged attacks of Rain's other Enhanced Armament (her magnum) (or her Incarnate, I don't remember) and Leopard's Bloodshed Cannon (not really a ranged attack but whatever), and then Pile and Crow follow it up with an Incarnate-powered Lightning Cyan Spike and a Laser Sword attack.
After all of that, cracks did start to form in the glass protecting the Armor's eyeball, but it didn't break. However, the Armor has been stunned, so Lime Bell immediately begins to use her Special, Citron Call Mode II, in order to get Rain's Enhanced Armaments off of it and back into her hands. It requires ten seconds to do the whole thing, and it's about to work, until the Armor suddenly starts moving again. Crow yells for everyone to keep it in place, but then it actually transforms into a vehicle mode - just like Rain could do with the Invincible - and suddenly flies straight at them at top speed, breaking out of Citron Call. No one can really do shit about it.
Crow tells Pile and Bell to go and find some buildings for Bell to break in order to fill up her gauge up again - he, Rain, and Leopard would handle the rest. They go off to do that, and the Armor turns around to face them again. Sixty seconds is up, too, and it decides to fire the main cannon a third time. Rain and Crow dodge it and close in on the Armor to attack from close range, but their attacks don't really do much, and the Armor starts driving at full speed with the two of them on it.
Suddenly, the two arms of the Armor appear and grab Rain and Crow, in one hand each. Crow tries to lift the whole thing in the air, but he can't actually deploy either of his sets of wings in this state. Then, the Armor literally starts smashing Crow and Rain together, and they both go into the red zone in terms of HP. Crow struggles as hard as he can, but Rain announces that she'll create a diversion somehow and wants Crow to take advantage of it and escape. Crow, of course, will have none of that - dying in a place like this would almost certainly mean they'd be EK'd. But there seems to be nothing he can do about it. He tries as hard as he can to fly the Armor up into the air (in order to drop it, ala Wolfram Cerberus), and he actually makes some headway, since he squirmed his way just enough to deploy his wings. But the thing is heavy. As he struggles, his desire to go faster, faster, faster, faster strengthens. The Armor is about to fire the cannon at point blank range at him and Rain, but suddenly... he hears the Acceleration noise.
Yeah, he hears the Acceleration noise after already being in Acceleration.
He suddenly appears in a completely dark world - he can't see a single thing, and he doesn't even seem to have a body. His soul is just kind of floating in an infinite, starless blackness. He starts to panic, and calls out the names of everyone he was with. As he descends further into despair, he finally says Metatron's name. When he does that, a tiny ball of light appears before him. It turns into a halo, then turns into a full-size version of her icon that was following them around before - her true second form.
After she gets mad at him for not saying her name first and foremost, and yells at him for trying to touch her even though he's her servant, Haruyuki asks where in the hell they are. Metatron tells him to expand his consciousness, to concentrate and try and become "aware". He does so, and manages to see a very blurry image of what's happening at this very moment - he and Rain are in the Armor's hands, and the cannon is charging up. He is also able to regain his form as Silver Crow instead of just being a soul.
It seems like it's a still image, but Metatron tells him to look more closely. Haruyuki realizes that the energy particles charging in the cannon were moving extremely subtly, and Haruyuki realizes that somehow he's accelerating far faster than normal. Metatron tells him to expand his awareness even further, and she takes his hand and they fly "upwards". After being enthralled by Metatron's figure for a moment Haruyuki gets back to focusing on the task at hand, and he starts to see more and more of the world below him.
They go up even further, and he is able to see quite a bit of the area they were in. In fact, he starts seeing small lights everywhere. He realizes that these represent Duel Avatars, enemies, and portals, and each of them "feels" slightly different. Then, Metatron says that they're on what she is calling the "Highest Level", a level above the world that Metatron lives in, the Unlimited Neutral Field. Haruyuki then sees the lights representing Lotus, Raker, Maiden, and Current, and realizes they must have figured out something was up, which is why they're heading their way. He can also see the lights of Enemies - er, Beings - all converging on the Armor's present location due to all the Incarnate being used.
She tells Haruyuki to expand his awareness once again. Suddenly, he starts seeing far, far more lights - but these are all different. He suddenly realizes what they are - they are all Social Cameras.
They look down at the Twilight Stage-ified Tokyo's Social Cameras, and Haruyuki notes that (as some people theorized) there are far more cameras than are visible normally - a lot of them are hidden out of sight for security purposes. However, their view settles on the Imperial Castle, which is completely dark: no social cameras. After noting that, Metatron makes Haruyuki a deal. In exchange for him "giving" her his memories of his time in the Imperial Castle, Metatron will explain everything she knows about the Accelerated World - including its purpose. Haruyuki agrees (he wouldn't lose his memories, just show them to her basically), so Metatron pushes her head up against his visor. The memories rush through his head, and Metatron pulls away, having seen it all.
She again says that they're on the Highest Level, which is higher than the Mean Level, which is apparently the Unlimited Neutral Field. However, she explains that the darkness of the Imperial Castle is extremely interesting. Haruyuki doesn't understand why - of course there's no cameras in there! - but what Metatron shows him next makes him realize it.
The "world" underneath them suddenly becomes a single plane, and two more planes appear, above and below it. These planes look like the Tokyo metropolitan area as well, and they are all connected by rays of light representing the Social Cameras. However, Haruyuki doesn't see any other activity in the other two planes - no lights for people, for NPCs, or for anything.
Things click in Haruyuki's mind at that point, and he asks if those are the worlds of Accel Assault 2038 and Cosmos Corrupt 2040. Metatron says, yeah, but she's surprised she knows what they are - Haruyuki explains that Green Grandee had told him about those other "trials". Metatron speculates that Green Grandee has also been on the Highest Level before. But anyway, she again explains that those worlds have been completely shut down.
She points his attention once again to the darkness in the middle - it's the same for all three worlds. Metatron then tells Crow that she thinks that the actual goal of not only Brain Burst (BB), but also Accel Assault (AA) and Cosmos Corrupt (CC) is actually the Imperial Castle itself. However, this presents an inconsistency. First of all, Azure Heir (Trilead Tetraoxide) was in the Imperial Castle. Second, the seventh of the Seven Arcs, The Fluctuating Light, is apparently the ultimate prize of that "dungeon". If the developer's original goal behind Brain Burst was the Imperial Castle, then how did an item that he or she should have created originally get placed in there?
Metatron isn't sure. She guesses that the ultimate goal probably has to do with The Fluctuating Light itself, and it being guarded by enemies that are even stronger than the Four Gods. Metatron says that in all the 7~8000 years she's been alive for, she's essentially become self-aware, and her goal, like Crow's and Lotus's, is to see the very ends of the Accelerated World. Haruyuki is quite shaken that an AI program representing an enemy character has become self-aware. Metatron however says that they need to leave soon - spending too much time in the Highest Level apparently makes it difficult to get back, and she once spent something like three hundred years trying to get out this one time.
But Haruyuki is discouraged. How can he possibly do anything at all in his current situation, even with Metatron's help? Metatron says that he needs to shed his armor - the armor that is his Duel Avatar. After a bit of thinking and stuff, and with Metatron's kind help, he realizes it; what she really means is the armor around his soul, the walls he's erected to keep his heart safe. Metatron gently does the same, and takes his hands. Of course, for Metatron this means her clothes start to disappear. As Haruyuki tries his best to tear down the walls around his own heart, he realizes something - something about the day he originally became a Burst Linker.
He recalls the nightmare he had on that night. He finally realizes that it wasn't the Brain Burst program that was showing him that dream - it was actually himself. It was his own heart, and his own Incarnate, that created the nightmare. Burst Linkers are all born using Incarnate from day one. Depending on the person's imagination, their soul, their desire - they are assigned a color and a noun. Red for those who desire the strength to reach faraway places, blue for those who desire the strength of their own body to overcome obstacles, green for those who desire the strength to protect their dear friends - and metal for those who desire not strength, but rather the power to protect their own heart from harm.
As they fade out back into the Mean Level, something interesting happens in the Brain Burst's central server. The writing here was complicated and I didn't quite get it, but essentially, the "stars" representing the Duel Avatars apparently also represent enemies. And while the ISS Kit's main body was connecting those stars via negative energy, Silver Crow's star and Metatron's star formed a link (first ever?) via pure positive energy. (There's some other weird stuff here too, like the server itself being affected by Incarnate or something but I'm honestly not sure.)