Magic God Invasion Arc

The Magic God Invasion Arc (魔神襲来編) is a story arc in Shinyaku Toaru Majutsu no Index, covered in Shinyaku Toaru Majutsu no Index Volume 13. During the Anti-Crime Orientation, Kamijou Touma is approached by the High Priest on behalf of the weakened True Gremlin. After refusing the Magic God's offer, Touma ends up fleeing from the Magic God on an Acrobike, together with Misaka Mikoto, leading to a destructive chase through Academy City.

Background
The Magic Gods of the true Gremlin had been living in a Hidden World, a phase separate from the rest of the outside world, and were content to remain that way. However they had concerns about distortions caused by their immense power and existence, which would influence the outside world, even if they didn't intend to. In order to address this, they wished to obtain a "scorer" who would direct them and their power and regulate the distortions, giving them piece of mind in exchange for having their wishes granted. The individual they intended to be their scorer was Kamijou Touma, bearer of the Imagine Breaker, a power born from the wishes of all magicians, with the Magic Gods being the largest influence.

After intervening to prevent Othinus's death and a possible undesirable change in Touma, Gremlin's hidden world was breached by Aleister Crowley who engaged them in battle, eventually retreating with a third of his body badly burned but obtaining the Magic Gods' base parameters. Following the loss of their hidden world, Gremlin decided to take action themselves and using a spell devised by Zombie in order to enter the world without risking destroying it, the Magic Gods entered Academy City on December 1st. The incident in the Dianoid with St. Germain delayed them, but as they were about to make a move, they were contacted by Aleister, who swapped out Zombie's spell with his own spell calibrated from the parameters he'd obtained in the previous battle, greatly weakening them. A crucified Zombie was then launched into the Magic Gods' midst by Kihara Noukan.