In earlier some earlier sections where Frisk can be hit by projectiles, they could immediately take damage similar to Deltarune instead of initiating a "mini-encounter," except the SOUL doesn't appear. In null_4, both Frisk and Chara's SOULs glow when Papybot advertises the exploding Pipis. Where does Pipis come from? Spamton. Which game did Spamton originate from? Deltarune. What did Deltarune introduce? Overworld dodging with the SOUL glowing.
So why does Asriel have a giant church dedicated to himself and Chara, designed to showcase his self-aggrandizing hypocrisy? Simple. From a certain point on view, many Bible verses in both Testaments portray God as someone who should always be obeyed, who is always right and knows best and who is considered kind and all loving; all of which can be interpreted as God being an abusive patriarch who gaslights people into obedience and uses emotional manipulation to decisive others in seeing them as sympathetic and benevolent, much like how Asriel is trying to keep Chara in an abusive relationship by putting them on a pedestal while still having all the power in their friendship.
Another bit regarding Asriel and his Rift Cathedral: Asriel is named after the Angel of Death which is consistent with his actions in Inverted Fate (i.e he absorbed everyone monster in the Underground and did the same with several humans on the surface.) While Asriel maybe named after the Angel of Death in terms of function, he's more consistent with Lucifer aka Satan due to him being a monster associated with goat-like imagery who creates illusions to deceive people and his status as a Fallen Hero. The Effigy of Ego is Asriel's god complex given form, and a god complex is a sign of extreme pride, the sin associated with Lucifer.
Unlike the Effigy of Ego and Pawn of Possessiveness, it makes sense that Fiend of Fatality would be defeated through kindness rather that calling them out. The Fiend represents Asriel's child self with it's more aggressive nature being represented by Chara's past self. The other Figments represent Asriel as he now as a fully grown adult.
It's eventually revealed that in one of Flowey's previous runs, he found and killed Alphys and the monsters she evacuated to the True Lab when he was killing everyone, something that the player couldn't do in Undertale. This is likely because unlike the player, he could burrow through the ground and reach the True Lab even if it was locked.
The Entity of Emptiness insists that sparing them won't work yet gets angry when Frisk and their party decide to spare them repeatedly. This goes two ways: The Pawn of Possessiveness got upset when Frisk and Chara refused to engage with it by running away, implying that Asriel doesn't like being ignored. Additionally, Flowey's motivation for doing all those playthroughs was because he wanted to see something new and interesting. By doing nothing but sparing the Entity of Emptiness, Frisk and the party defeat the Entity by deliberately doing the same thing and ignoring it, causing the entity to became exasperated at them constantly spamming the Spare button every turn.
Unlike the other Figments, the Entity of Emptiness is Defiant to the End, spitefully telling it's master that his plan is doomed to fail and that he'll always be empty. Then you remember that this particular Figment is based off Flowey, and seems to have enough of his personality that as a result, he's slightly more self aware than the others and just as spiteful and cruel as the real deal, much to Asriel's fury.
Some optional dialogue in the playable null_5 reveals that there is a statue of Chara on the surface, implied to be a memorial made by their birth parents. This means that not only are other humans likely to recognize Chara like Lilac did (and possibly misgender them/call them by their deadname), but also that some of them are aware of the existence of monsters and believe that Chara was killed by them, which might cause problems after the other monsters are brought back.
When Frisk, Chara and Lilac finally escape from the corrupted Underground and make it back to the city, we see a few bodies drained of life, only moving to point the heroes in the intended direction. It's likely that what we saw in null_7 is only a small fraction of the horrors that Asriel's plan has inflicted. Additionally, Asriel's plan has warped reality to the point where several Gaster followers (aside from Spamton's cameo in the Cyber City building)are able to exist out in the open.
Though Word of God has confirmed that Asriel has no plans to become a Multiversal Conqueror, the fact that Asriel is powerful enough to see into other universes means that he's able to learn from them to see where he went wrong in other universes, allowing him to cover whatever weaknesses he has and make himself ever stronger.
If Asriel was a child when he first became Flowey -a creature devoid of love- who would eventually do multiple playthroughs just to see every possibility, then it's possible that Flowey spent years in the Underground in the same location. That would be more than enough for Flowey to start seeing everyone as NPCs who engage in predictable fashions, and it's likely that Flowey had snapped after several playthroughs in and decided to kill everyone in the vain hope of feeling something after years of dehumanization, derealization and emotional numbness.
