WARNING: The following contains spoilers for Episode 64 of Digimon Adventure 2020, "The Angels' Determination," now streaming on Crunchyroll.
After the Holy Digimon, HolyAngemon/MagnaAngemon and Angewomon, transformed into Holydramon and Goddramon in the final fight against Millenimon, it was only a matter of time before they also became able to evolve into their other Mega forms, too. Episode 64, "The Angels' Determination," finally provides this payoff by pitting the new Celestial forms against an adversary of contrasting demonic darkness: Deathmon.
Deathmon is formed in the episode out of an army of Soundbirdmon, who are attempting to destroy the Chosen Ones' crest tablets hidden safely within Wisemon's tree. Also known as Ghoulmon, the huge monstrosity is one of the Demon Lords, a group of powerful, fallen Angel Digimon, similar to the way Devimon was once of the same species as Angemon. Speaking of the devil, the former Big Bad of Digimon Adventure (both the original series and current reboot) also makes a cameo in Episode 64 in the minds of both Takeru and Patamon, reminding the latter of the possible fate that could one day befall it, as well as warning them what Deathmon's arrival heralds.
Deathmon is essentially like one of the Four Horsemen of the Apocalypse -- this apocalypse being the Great Catastrophe that is Negamon's birth at the episode's end. Unfortunately, this eventuality means that even defeating the Demon Lord can't stop what's already been putting in motion, with its mere presence enough of an omen to cast doubt into all who watch on in the Digital World of the Chosen Ones' abilities to prevent disaster from striking.
In spite of this omen, WarGreymon, Holyangemon and Angewomon stand resolute in their defence of the crests and the Digital World at large. After Deathmon blasts a sizeable hole into the tree trunk, Hikari and Takeru trigger their respective partner's evolution sequences, making Ophanimon and Seraphimon appear in the reboot for the first time. As Celestial Digimon, and the final forms of their male and female-presenting angelic lines, both are on par with the Demon Lord's power, if not more so, which they prove by doing to Deathmon what it did to Wisemon's repository of digital knowledge.
While we know very little about what Negamon is, Devimon does clarify exactly what the mysterious, evil entity wants, at least -- the complete annihilation of the Digital World, down to the last pixel. When the archvillain is done, Devimon tells Takeru during a dream sequence, not even "despair will remain." All the more important, then, that the Chosen Ones have a pair of Celestial Digimon on their side. Not to mention, Ophanimon and Seraphimon are two parts of the six Mega Digimon that can fuse to form Omnimon Merciful Mode, which hasn't been used in the anime before.
In the previous episode, Valkyrimon heavily hinted to Tai that Ominmon is the "miracle" needed to prevent such all-encompassing destruction. Miracle or not, if Negamon really is a threat unlike any other in Digimon Adventure, the warrior will certainly need the extra fusion power a new form like Merciful Mode can provide to hope to stand a chance.