WARNING: The following contains spoilers for Episode 63 of Digimon Adventure 2020, "The Crest of Courage," now streaming on Crunchyroll.
After weeks of meandering around the Digital World and helping out his friends with their Crest quests, Digimon Adventure 2020's Taichi (or Tai) finally gets to embark on his own in Episode 63. "The Crest of Courage" tests his Crest's attribute in order to strengthen his and Agumon's bond -- a requisite for evolutionary purposes -- and also reveals some important insight into the role of Agumon's most powerful form in the series: Omnimon.
Omimon is the fused form of Agumon and Gabumon at the Mega level (WarGreymon and MetalGarurumon). Debuting in Digimon: The Movie, the now fan-favorite fusion warrior has made a few select appearances throughout the years in subsequent franchise installments, with the currently airing Adventure reboot being the most recent. After a surprisingly early introduction in the series, however, Omimon has only appeared (flashbacks non-withstanding) once more. Episode 63 explains that there's a good reason for that.
In the episode, Tai is essentially sent on a vision quest without Agumon, designed to put him back through some of the trials he's encountered along his journey in the Digital World -- but this time, without the support of his Digital companion. Instead, a totem version of Agumon is relegated to a freshly-hatched stage and entrusted to Tai to protect as they alternate between fire and frying pan situations. As brave-faced as ever, Tai pulls through and is rewarded by a history lesson from his quest-giver -- a mysterious, glowing Digimon who isn't named in the episode but identifiable as Valkyrimon.
Valkyrimon reveals that it was part of the Army of Light, which, thousands of years ago, helped destroy Milleniumon. The final nail in the coffin for the villain was WarGreymon and MetalGarurumon -- with some help from Orphanimon and Seraphimon, transforming into Omimon -- who Valkyrimon describes as the "miracle" they needed to turn the tides of war. This trip down memory lane is meant to impress upon Tai how instrumental Omnimon could be in stopping the Great Catastrophe, the latest threat to the Digital World's existence in the present day.
Valkyrimon also alludes to what it takes to summon Omnimon, which explains why it has seldom appeared in Digimon Adventure 2020 so far. To viewers and those who know their Digimon lore, the answer probably seems obvious without this context: Omnimon is your basic 'Final Form' anime archetype and, as such, shouldn't just be brought out against anyone and everyone. Doing so would detract from its specialness and make the foes it fought seem like less of a big deal, too. You don't bring out the big guns for mere grunts.
But Tai, not knowing he's an anime, of course, needs an in-story reason, which Valkyrimon provides: Omnimon, being a miraculous being, is very hard to make appear, requiring a tremendous amount of heart and exertion to do. Fused forms are doubtless rarities in the wild and largely lawless dimension that is the Digital World, the inhabits of which would often prefer to work against one another than with, or, with some exceptions, don't actively have reasons or yearnings to become stronger to such a degree.
Omnimon's reemergence has been on the cards for some time now, with each of the gang's Crest missions enabling their Digimon to reach at least one form of Mega evolution by now, possibly teeing up an all-new power-up for the "miraculous" weapon. The preview for the next episode of Digimon Adventure 2020 also teases Seraphimon's arrival, the Mega form of MagnaAngemon/HolyAngemon. Given that we now know the key part the Holy Digimon played eons ago in Omnimon's appearance, there's no question that we're closer than ever to seeing the iconic Digimon again.
