Null session

A null session in Sburb is any session of the game that is doomed to failure.

While this refers to any session that fails to create a universe, it also seems to refer to sessions where the players create a universe but are still unable to win. Both Karkat Vantas and Aradia Megido HS 4467 (call their) HS 4509 (own session) a null session after Jack Noir prevented them from obtaining the ultimate reward, even though they successfully created a universe and killed the Black King.

According to Rose Lalonde, null sessions HS 6601 (are actually very common); she notes that redundancy is a property to be expected in any reproductive system, and universes are no exception. As Skaia is "aware" of its own fate, null sessions are doomed to fail by design; the Scratch can offer a second chance of sorts by resetting the universe, though the new session that results may also be null.

The Session Glitch is hinted to create a null session.

Void session
A HS 6601 (void) HS 6602 (session) is a specific type of null session where none of the sprites are prototyped pre-entry. On the one hand, this prevents the underlings and monarchs from gathering any significant powers from prototypings. On the other hand, such a session cannot be won as the Battlefield will not reach its final form, preventing the creation of a new universe. The players will be doomed to live out the rest of their lives in the session.

There are a few traits unique to void sessions that are encountered by the post-Scratch Kids. In a void session, both Prospit and Derse lack prototyping towers, as Skaia is already aware there will be no pre-entry prototypings. Also, lacking any pre-entry prototypings, the Underlings encountered in void sessions are simply skeletons, which are highly difficult to kill and produce almost no grist. Lastly, the Reckoning does not occur in such a session, as the White King never falls in battle due to the everlasting Stalemate that occurs if nothing is Prototyped.

The players of such a session can be referred to as Nobles instead of Heroes, because under normal circumstances such a session is slated to end in failure and the players cannot fulfil their heroic destiny. This is also a chemical reference, as the noble gases cannot react naturally, just as this session cannot be completed naturally.

All the consorts of each planet in the post-Scratch kids' session had died out by the time they arrive (though their denizens still live), and we HS 6720 (read) that "The Aspect of Hope was dead on arrival"; however, it is unclear if this is a common trait of a void session or an anomaly specific to their session.

Dead session
A HS 7397 (dead session) is a sub-set of void sessions, it is the result of a player attempting to start a single player session. There is no hope of creating a universe in such a session under any circumstance, not even with outside help. Dead sessions have been described as "" to other sessions, with victory and defeat being dictated by totally different terms. Despite this, victory is still designed to be impossible to achieve as the session is meant to punish a player for their hubris in attempting such a session, only one player is known to have actually succeeded.

Prior to the session being launched, Skaia becomes permanently covered by dark clouds. When the session is launched only the Cruxtruder is able to be deploy, which releases the kernelsprite which collapses into a black hole, sucking the planet into it, and eventually the sun and entire solar system.

The only denizen of a dead session is Yaldabaoth, and The Choice he gives is to bring the actual Land of X and Y to the dead session's only player, allowing them to begin the real game, however, it becomes very difficult for them. Despite having only a single player, a dead session can contain multiple planets beside's the player's home planet. The goal of such a session appears to be conquering/destroying numerous planets in the session in a certain amount of time and unlocking minions. The ultimate goal of such a session appears to be to eventually defeat Yaldabaoth in combat and claim a reward of unconditional immortality and be given infinite power to destroy anything.

It is considered the extreme difficulty of Sburb.

Examples
Every session seen or heard about in Homestuck is some kind of null session.

Although the precise circumstances are unknown, the players of pre-Scratch Alternia were in a null session due to the Session Glitch and the presence of a Tumor.

The troll players of post-Scratch Alternia, as previously stated, were successful in creating a universe. However, it seems their session became null after Jack Noir prevented them from entering the newly created universe. It is implied that and  had deliberately engineered this result through their subtle machinations and corruption of troll society.

The pre-Scratch kids also had a null session. It seems that this is the result of Karkat giving their universe cancer by failing to properly breed the Genesis Frog in his session. This cancer manifested as Jack Noir, who ruined the session by killing the Black Queen and Black King before the Reckoning, and as the Tumor, which created the Green Sun.

The post-Scratch human session is intrinsically a void session, but this may be fixable. Jane Crocker did not prototype anything before entering her session (due to interference from God Cat), and neither did the other players (due to more pressing matters, such as escaping the Miles). Even if their session was not circumstantially void, it is evident that their session would have been null regardless due to the fact that they have no Space player and therefore no one to breed the Genesis Frog (nor a planet where the frogs would be found) and create their new universe. However, Jade may be able to fix this, as she is a Space player who is in possession of a fully prototyped Skaia, as well as a planet which is inhabited by frogs.

The cherubs' session played by Caliborn is a dead session, as Prospit is shown not to have any prototyping towers much like the Prospit and Derse of the post-Scratch human session. Storm clouds had begun to grow around and eventually completely surround Skaia, there are no gates leading to Skaia, and the Prospitians and Dersites were shown to be wearing black and white respectively rather than the usual opposite; it is unknown which if any of these properties are a result of the session's status. The cruxtruder was the only device available to deploy, depriving Caliborn of the ability to alchemize anything, although he may have unlocked alchemy equipment on other planets as he is later shown wearing a customised Caltop helmet. In addition, his quest involved unlocking and conquering new planets, each of which granted him a leprechaun-like minion who are presumably the past versions of.