Skaia (a portmanteau of the words "sky", "kai", and "Gaia") is the world that the players of Sburb must build toward in order to win the game. Nanna describes Skaia as a "dormant crucible of unlimited creative potential". It resides at the core of the Incipisphere, sort of like its "sun".
An orbiting light planet, Prospit, is charged with its defense, while the distant dark planet of Derse covets its destruction. The Armies of Light and Darkness duel there in a perfect stalemate until a player prototypes a Kernelsprite. Then, their real battle begins, one that Prospit is destined to lose.
The planet at the center of Skaia is known as The Battlefield, which hosts the war between the forces of Prospit and Derse. The Battlefield is portrayed as a chessboard.
Each time a Sburb player initially prototypes a Kernelsprite, the size of the board seems to grow exponentially: Where The Battlefield is initially a three by three chessboard (a situation which leaves the only two combatants, the White King and Black King, in an eternal stalemate), the board grows into a full game of chess after a first player's prototyping, albeit with a twelve by twelve square board and some pieces taking up multiple squares.
The chess-like battlefield.
When a second player prototypes his or her Kernelsprite, the board expands to a massive cube with forty-eight squares to an edge and landmarks like trees and lakes. The Battlefield becomes a fully spherical planet upon a third protoyping. After the fourth prototyping, The Battlefield appears to have gained a large outer shell made of checkered tendrils.
The Battlefield of a 12x-prototyped Skaia is shown near the end of Homestuck: Act 5 Act 1, but the full planet is never shown. Before the battlefield is prototyped to its full size, Skaia's core consists almost entirely of a sky dense with vast, twisting clouds, much more than has been seen in the inside after prototyping.
The final form of Skaia is required to create and grow the new universe, fulfilling the objective of the game. This is why a pre-entry prototyping is necessary for each player. According to Rose, the prototypings enable Skaia to transform and heal itself, thus letting it "handle" the aforementioned new universe. She also mentions a "treasure" that is in the center of the kids' version of Skaia when the final prototyping occurs (which may be unique to the kids' session, or failed sessions in general).
This treasure is referred to as The Tumor. Similar to other powerful factors in the scope of Homestuck (such as Doc Scratch) it is written in a color other than that usually used by characters - in this case, black. It is later revealed to be an amazingly powerful bomb, most likely capable of blowing the entire incipisphere out of existence.
A Dersite castle.
Usually when the Black Kingdom inevitably wins the battle, the Black King uses the White King's scepter to initiate The Reckoning, sending all the meteors in The Veil towards Skaia to destroy it, but in the kids' session Jack Noir takes the scepter and does it himself. Skaia defends itself by opening portals to stop the meteors, sending them instead to the host planet and sacrificing the planet to protect itself.
By this means, Skaia forces players to enter the game in the first place. Skaia eventually fails to protect itself when the largest meteors in the Veil are launched at it, at which point it is destroyed, due to either running out of defense portals or an inability to teleport objects of such size. The players' goal is then usually to stop the Reckoning before this happens by defeating the Black King, thereby winning the game.
The players are to then deploy grist rigs at the top of their builty up houses and disperse their planets' grist hoards into Skaia, preparing it for the Ultimate Alchemy, while one of the players lights the forge.
In addition to Prospit and Derse, one planet for each player in Sburb orbits Skaia, separated by a series of gates. Also, just inside of Derse's orbit is The Veil.
A Prospitian castle
When a player visits Prospit as his or her dream self, sometimes Prospit's moon eclipses the oracle clouds
of Skaia. These clouds float by the player's dream tower and portray glimpses of the past, present, and future to him or her in cloud mirages. These clouds are also visible on the planet itself. They are capable of showing virtually anything, even things from other incipispheres or wherever Andrew Hussie is. Prospitian (and possibly Dersite) dream selves who reach god tier are transported to a copy of their Quest Bed on the battlefield.
Many times during the game, Skaia is referred to as a sort of sentient being. Rose also states that it is a "passive being" at that, only "watching" and "seeing" instead of "acting".
Skaia's name is a portmanteau of "Sky" (possibly from the Greek god Ouranos, who ruled the heavens) and "Gaia" (The Greek Being of the Earth). According to the Greek creation myth, these two beings gave birth to the titan Cronus, who had dominion over time, as well as the rest of the titans. It could also be a reference to "the Skaian Gates" in Homer's Iliad, from which Priam watched the Trojan War. Seeing as the children must travel through The Seven Gates, and also the portals which send the meteors to Earth are 'gateways' of a sort, the name is particularly fitting.
The chessboard that the Wayward Vagabond creates in Can Town in the Skyship Base is a symbolic representation of Skaia. In his game of chess, the Black side wins, much as Derse defeats Prospit on the Battlefield.
Gallery
Human sessions
Pre-scratch
Jack arrives on Skaia.
Even before any of the Homestuck Kids play Sburb, Jade Harley is able to visit Prospit during her dreams and gain an understanding of future events (much to the confusion of her friends). She also sees visions of the past, including one from 413 million years ago that shows a meteor striking what will later become her island. This meteor brings Becquerel and a "seed", a white object with a spirograph on it that eventually spawns a Frog Temple.
Skaia is now mostly in ruins from the meteors.
After war begins on The Battlefield, the Warweary Villein, a Dersite farmer who resides on Skaia, grows tired of the war and unites soldiers from both kingdoms to rise up against the Black King. Jack Noir murders the Black Queen, obtains the White King's scepter, and starts The Reckoning well before it is intended to begin.
He uses the powers gained by the Queen's Ring to ravage the Battlefield and murder all of the Black and White soldiers, excluding the Warweary Villein. He then severs the massive chain connecting Prospit to its moon, sending the moon crashing down to the Battlefield and killing Dream Jade in the resulting explosion. The Battlefield is left in ruins and with a massive crater.
John's dream self wakes up on The Battlefield, retrieves the White Queen's ring from Dream Jade's body, and sees his immediate future in the clouds. He is assaulted by Jack Noir, who seeks the Ring, but the Uber Bunny comes to his protection.
Rose mentions that the "treasure" known as the Tumor, located in the center of Skaia, is critical to her plans; it is later revealed that she plans to destroy the Green Sun with it, taking the power from Bec Noir and all of the first guardians. She also mentions that John must retrieve this object using The Windy Thing.
In Cascade, Jadesprite merged with Jade and rescued Skaia from a humongous meteor using her space powers. Jade then shrunk the inner portion of the Battlefield and took it with her to escape the scratch, leaving behind the "shell" section.
Post-scratch
Skaia's interior during a null session.
When the kids orchestrated the scratch, past Skaia manipulated the defense gates so that the meteors bearing the kids and their guardians would land in different places and time periods, so that each kid-guardian pair would be switched (John/Nanna, Rose/Mom, Dave/Bro, Jade/Grandpa). She also states that Roxy and Dirk living in "the future" instead of the time period Rose and Dave lived in was one of Skaia's "peculiar whims".
Rose also explains the kids' plans to deposit the pre-scratch Battlefield into Skaia, where it would supposedly produce the new universe. This is because the post-scratch session is a void session, wherein there is no pre-entry prototyping destined to happen. Skaia would remain in eternal stalemate, and the Black King would never initiate the Reckoning. Also, the Courtyard Droll was last seen blowing up the Black and White Kings (or their symbolic representations), making it all the more necessary for another Battlefield to be deposited.
Troll sessions
Post-scratch
The trolls' Sburb session was fairly regular, although they faced a 12x prototyped Black King able to release the Vast Glub, making it an unusually difficult final boss.
Despite being prototyped 12 times, Skaia's battlefield does not seem to differ from the kids', save for the lily-pad like structures. It should be noted that only the first 3 prototypings seemed to affect the battlefield itself in the kid's session, while the 4th added an additional structure surrounding it.
It is implied that, upon winning the game, the trolls' Skaia underwent a transformation. From the outside, it seems brighter than usual. It possesses a shimmering surface that resembles a spherical pond more than a sky, and multi colored glowing Lilypads are found on its surface.
The Genesis Frog is shown to be floating in the center, which implies that the session's Genesis Frog will be transferred to Skaia for it to grow, with Skaia functioning as its “crib”. It is suggested that pre-entry prototypings are required for Skaia to transform to support something as large and complicated as a universe. A platform bearing the Sgrub symbol has also appeared, to allow the trolls' entry into the universe they created and “rule it as gods”.
Of course, this was all interrupted by Jack Noir, who destroyed Skaia and the Genesis Frog within it.
Pre-scratch
The pre-scratch troll's Skaia was shown briefly while they activated their scratch. It displayed both the symbol of Time as well as Space while being scratched, much like the pre-scratch kids' did when scratching.
Cherub session
Skaia filled with dark clouds
In Calliope's and Caliborn's session Skaia has started to fill up with dark grey storm clouds, and is completely filled with them by the time Jack kills Calliope's dream self. This may be related to their game being a dead session, but it remains unaddressed. Curiously, in this session Prospitans wear black clothes, and Dersites wear white, an inversion of the sessions observed so far. This could tie in to Calliope and Caliborn's Yin-Yang theme.
When Caliborn makes The Choice to take the path of the conqueror, Skaia becomes darker and heavier, turning into a solid sphere before exploding in an event known as the First Break. Fifteen new planets emerge from the blast and scatter around the Medium while the remaining Skaian matter collapses into a black hole at the center of the incipisphere. Three of the planets are swallowed by the black hole, leaving only twelve. As part of his quest, Caliborn uses powerful bombs to push each planet into the black hole, "sinking" them.