Destruction of Tree Diagram

The Destruction of Tree Diagram is an event that occurs near the end of the Index Arc, in which the supercomputer Tree Diagram along the the Orihime-1 Satellite was destroyed. Even though information on the true fate of Tree Diagram is only known to a select few, it's effect is widespread throughout Academy City in both Toaru Majutsu no Index and Toaru Kagaku no Railgun storylines.

Tree Diagram

 * Main article: Tree Diagram

The Tree Diagram is the most advanced and powerful supercomputer in the entire world. Installed on the Orihime-1 Satellite, it was primarily used for weather monitoring with extremely accurate results; so much so, as it can actually schedule what weather condition would occur at a specified time in Academy City.

However, being the most powerful supercomputer it is often used for calculations, though only with the appropriate clearance from authorities, as such researchers like Kiyama Harumi cannot simply use it, and was denied several times for it's use. As is the power of Tree Diagram that was extensively used in the Level 6 Shift experiment, where it calculated that only Accelerator could achieve SYSTEM, and that he would need to kill Misaka Mikoto, the Railgun, 128 times or 20,000 of her clones. As such Tree Diagram is extremely valued by Academy City, and is the seat of Academy City's dominance over the world's technology.

Index Arc
A few mintues after midnight, Kamijou Touma is forced to fight Index in Johann's Pen Mode after activating the church's failsafe, Necklace, when he negated the spell that would kill Index if her memories were not erased. During the battle Index used Dragon's Breath on Touma; it is a spell that resembles a destructive beam of light, and it's after-effects were feathers of light that would doom anyone it touches. After initially forced to fend off her attack, Kanzaki Kaori, uses her power to make Index go off-balance. This makes the still in-effect beam to fire skywards, destroying Tsukuyomi's roof, it eventually reaches space and destroys Tree Diagram, which coincidentally was right in its path.

Aftermath
It's affects were widespread in Academy City, as people have become used to the scheduling of the weather by the computer, they are now forced to be more wary of the weather. However, the destruction of Tree Diagram itself has affected the whole world. Many countries with space programs have increased their launches into space, seeing the potential to show-off their technology and to steal the remaining pieces after it's destruction.

It also becomes a pivotal in Misaka Mikoto's decision to sacrifice herself for her clones, as she discovers that Tree Diagram has already been destroyed, and has not changed the fact that her clones are still being killed by Accelerator for the Level 6 Shift.

The remaining pieces is also central in the 8th novel of Toaru Majutsu no Index, as several factions try to obtain them, and forces Mikoto to try to destroy it, believing that the experiment on her clones may continue. Mikoto's involvement also coerces Shirai Kuroko, a member of Judgment, into getting involved with the dark side of Academy City, as she duels with Musujime Awaki, who holds 'Remnant', the essential core to rebuild Tree Diagram.

'Remnant' was eventually destroyed by Accelerator as he fought Musujime after being requested to do so by Last Order.