The Tumor is an enormous explosive device found at the center of the fully-prototyped Skaia in the pre-scratch kids' incipisphere. It has a white and black half, shaped similarly to a taijitu. Like the Magic Eight Ball, it features a round window through which a triangular shape is visible, though in this case the triangle simply displays a countdown to the device's activation.
Rose initially speculates that the Tumor's purpose is to wipe out null sessions that have failed to accomplish the game's goal, detonating when the Reckoning ends. However, she also surmises that it might not be a common game feature and may represent a mutation specific to the kids' session.
The Tumor was at the center of a plan devised by Rose to kill Bec Noir by using it to destroy the Green Sun, Bec's source of power. This plan was based on information obtained from Doc Scratch, who claimed to have a stake in the outcome: destroying the Green Sun would also kill him, thus accomplishing his goal of summoning Lrd English. Rose was also assisted by the Noble Circle of Horrorterrors for unknown reasons, supposedly involving self-preservation.
As planned, John was able to retrieve the Tumor after fully understanding of his powers as the Heir of Breath. He then assigned the Wayward Vagabond, the Courtyard Droll, and Liv Tyler to deliver the Tumor to Derse, where Rose and Dave (both as their dream selves) were waiting for it. Meanwhile, Dave had been mapping a route through the Furthest Ring to the Green Sun with the help of the Horrorterrors. When Rose and Dave received the Tumor, they detached Derse's moon and Rose began flying it to the Green Sun. Due to a last-minute complication involving the Draconian Dignitary, Dave then joined Rose on the moon, and they both delivered it to its destination.
When the Tumor finally detonated, it was revealed not to be a conventional explosive device at all: In fact, its true purpose was to create the Green Sun, a fact conveniently omitted by Doc Scratch during previous conversations.
The two tubes on each side of the bomb hold the kids' universe, in red, and the trolls' universe, in blue. The former is eroding from Jack's Red Miles attack being used on Bilious Slick, while the latter is similarly activated when Spades Slick shoots down Snowman with Doc Scratch's deudly firearms. The universes seem to represent chemicals that mix to cause the explosion. This yet again plays into the theme of duality and the two universes being linked.
Another interesting note: the Green Sun itself is said to be
In the pre-scratch trolls' session, Meenah (on the advice of unidentified "monsters", presumably either the Horrorterrors or Denizens) set off a device that looked like a miniature version of the Tumor in order to kill them all so they would continue to survive as ghosts after the scratch was complete. Unlike the Tumor, the device also seemed to have a function similar to the Magic Cue Ball. Whether or not this means that all null sessions have some sort of device similar to the Tumor is yet to be determined. The fact that this also killed Meenah and Aranea means it is the only way to kill a god tier in a way that is neither Just nor Heroic.