The Reckoning is a twenty-four hour event in Sburb within Homestuck. As part of the continuing battle between dark and light, the Reckoning occurs when the dark royalty uses meteors from The Veil in an attempt to destroy Skaia. As part of a defense Skaia uses portals to transport the meteors elsewhere, specifically to the host planet of the Sburb session. Due to The Medium occupying a separate location in space time Skaia can send the meteors to various points in time of the host planet's history. Most of them arrive at the same time, namely during the time the players start their Sburb session, motivating them to enter the Medium in the first place, which creates a circular paradox. According to Karkat Vantas, the kids' session of Sburb had The Reckoning occur much earlier than it is supposed to in an average session, on account of Jack Noir.
In the pre-Scratch kids' session, after killing the Black King on the Battlefield and thoroughly wrecking the white and black armies, not to mention the planet, the Sovereign Slayer receives the White King's Scepter from the Courtyard Droll. Jack Noir then uses it to begin the Reckoning, sending the Veil towards the ravaged Skaia. As in the actual game of Chess, thanks to Jack Noir, white made the first move.
The Reckoning is an important part of the story of the Paradox Clones. At some point after being "born", each paradox clone is transported onto one of the meteors in the veil, and with it, through Skaia's defense gates, to the player's home planet, into the past, thus creating them in the first place.
After the Scratch is activated, the meteors carrying the paradox clones are sent to an alternate version of their home planet. Other meteors don't get this treatment.
Each of the clones from the kid's universe took with them an object or animal: Grandpa takes his revolvers, Nanna takes Dad's oil-stained fedora, Rose's Mom takes Mutie, Dave's Bro takes Dream Lil Cal, John takes the Colonel Sassacre book, Rose takes the original bunny, Dave takes Maplehoof, and Jade takes the Knitted Bunny. If that is just a coincidence or another constant of the game is unknown.