C'mon Digimon

Digimon Chronicle was the storyline accompanying the 2004 Digimon product line. Originally rumored to be a new, full-blown manga to replace V-Tamer 01, it eventually transpired to be mostly prose text, printed in the booklets which accompanied the "Pendulum X" digital pets. These booklets also contained short, non-sequitur six-page manga. There are four "chapters", one sold with the Pendulum X 1.0, another with the Pendulum X 1.5, another with the Pendulum X 2.0, and the final chapter with the Pendulum X 3.0. This fiction tells the story of a Digital World controlled by a scientist computer named Yggdrasil. Because Digimon had multiplied so much in the past, Yggdrasil, the host computer, was unable to handle the load and the Digital Hazard occurred. Yggdrasil then created the "New Digital World", consisting of three layers for the past, present, and future - Urd, Versandi, and Skuld, respectively, and then let loose with Project Ark and the X Program to eliminate any Digimon Yggdrasil no longer wanted. However, the Digimon adapted by obtaining a program called the X-Antibody, which strengthened them, changed their form, and made them immune to the X Program. Yggdrasil sent in the Royal Knights, thirteen Digimon devoted to keeping order in the Digital World, and at this point Kouta, Yuuji, and Shinji, three humans, somehow found their way into the Digital World and met their partner Digimon, Dorumon for Kouta and Ryuudamon for Yuuji. Kouta and Yuuji resisted Yggdrasil and the Royal Knights, but Shinji apparently sided with Yggdrasil. Source: mangaupdates

Tsurugi Tatsuno is a boy who competes in Virtual Digimon Battle tournaments. When a Kuwagamon appeared in the Real World, his Greymon came to life and protected him. Tsurugi is summoned to the Digital World by Piximon to save the Digital World from the evil Barbamon. Since his Digimon has a hexagon shape on him, it's an Illegal Digimon meaning that Digimon bearing the symbol can help save the Digital World. Tsurugi's Greymon later De-Digivolves to Agumon and they meet other characters destined to save the Digital World from the forces of Barbamon. Now they must stop Barbamon and his minions before they get all the DigiMemories and take over both worlds. (Source: Wikipedia)

It starts off with Taiki having a dream about leading a Digimon army with himself as its "General" against one consisting of several Mammon, Rhinomon and Valvemon. He then Digixroses Shoutmon, Balistamon and Dorulumon into Shoutmon x4 who effortlessly takes out most of the oppossing army. Oversleeping, Akari wakes him up with a "move" she calls "Akari Strike". After being encountered by Zenjirou, Taiki suddenly hears a melody from out of nowhere with Shoutmon crying out. Taiki starts searching for the source of the voice in order to save him. Chasing after him, Akari and Zenjirou witness a car about to fall on top of Taiki, but he narrowly escapes its path thanks to Shoutmon's warning. Suddenly, the buildings and surrounding space begins to distort, causing electronic disturbances. Afterward, Taiki discovers a light along with a melody, then a mysterious voice says "do you want to save this melody?" and a Xros Loader appears. After following further instructions from the same voice, and once the melody is absorbed by the Xros Loader, Taiki, Akari and Zenjirou fall into a wormhole-like digital space which emerges on the ground, transporting them to the Digital World. Once there, they are in the middle of the Bagura Army, the same army Taiki saw in his dream, lead by Mach Leomon.

A one-shot pilot manga, made in Summer 1997, involving battles between more-or-less holographic Digimon. However, this manga was published as a special in volume two of Digimon Adventure V-Tamer 01, where an omake page by the author explained that the hero of this manga, Kentarou Kamon, was the source and inspiration for the design and character of Taichi Yagami, the protagonist of V-Tamer 01.

Seven kids at summer camp are unexpectedly transported by 'digi-vices' to a colorful and enigmatic world. Lost and alone, they are befriended by small digital monsters, Digimons. The kids and their Digimon partners quickly become inseparable, and the strange creatures guide their human friends through the wondrous DigiWorld, protecting them from the hordes of evil Digimons. The group soon discovers that some of the giant Digimons they encounter are not evil Digimons but good Digimons gone bad. A dark power is corrupting even the most gentle of Digimons by embedding Black Gears into them, turning them into vicious monsters. Creating chaos and destruction wherever it goes, the evil power threatens all of DigiWorld. As they try to find their way back home, the seven kids are drawn further and further into the mystery. Through teamwork and trust, they help their newfound Digimon friends digivolve from 'monsters in training' to giant champions that must save the DigiWorld from the powerful evil intent on destroying it. (Source: TOKYOPOP)

Special one-shot between Digimon Adventure V-Tamer 01 and Digimon Adventure (2020).