After all three main routes, almost everything beforehand in terms of what is "remembered" has been given a clean slate, mostly though through the past genocide events. Frisk still lost the ability to freely control their soul due to finishing the latter route, but this needs to be done so that Chara's being/SOUL could fully manifest from just being "connected" to Frisk at the starting point.
In this mode, a pacifist route is a must. Killing anything will cause hard mode to be locked out again until you go back and finish both Pacifist and Genocide.
Erasure and SalvationAfter the God of Hyperdeath Boss fight, Frisk makes their way to the very beginning where Asriel is tending to the flowers on Chara's grave. They Begin talking with him until he eventually says "Chara wasn’t really the greatest person." This will causes a stir within Chara's conscious, which shifts to them to be in main control of Frisk's body.
At this point Chara is trying to mutilate Asriel but Frisk's conscious is trying everything in their power to stop them. The tension eventually becomes so immense that it causes Chara's being/soul to be split apart from Frisk.
Chara's soul, now separated from Frisk, uses all the Determination, EXP and LOVE acquired from the Genocide Route to create a partially complete form of their-self beyond the typical Heart Shape a soul usually looks like (So like a phantom, I guess). With Frisk, now completely exhausted and time running out for Asriel's temporary soul until he becomes a flower again, the final battle sequence starts.
Asriel can be saved if you manage to fend off and bring peace to Chara. (Even though there's a huge plot hole to the idea as to "how" Asriel was saved and regained a soul,) Flowey more-or-less is just a bad memory.
This battle revolves around a loose theory that Frisk, Chara and the six other human souls are all fragments of one singular being/soul/conscious/etc.
- Frisk and the others were a random anomaly of nature, with their only sole purpose being to help free the monsters. Now that their mission was accomplished, Frisk's determination is forcing them against their will to recollect the other souls to form "The Nexus".
- The Nexus isn't malicious or a threat to resetting the timeline. But rather it's trying to perform a reset on itself in order to erase its existence as a protocol to protect the monster's future. Kind of like restarting a game back on it's latest save point to get rid of a glitch.
- Obviously the gang doesn't want to lose Frisk. So they begin to do everything they can to save them, even if it means battling the Nexus.
- You'd be the monsters fighting against the Nexus in control of Frisk's body and the other human souls, including Chara. The current idea behind it is that this battle plays a little similar to Earthbound, where you control one character at a time to tackle certain events within the fight.
- The Nexus is kinda like a robot that follows Asimov's laws... but instead of humans, it's monsters. So essentially, it won't fight back. Which can make a bit of a boring battle... So while the Nexus begins gathering enough determination to make this reset, the beginning portion of this fight would more or less consist of many obstacles that the monsters have to dodge in correlation to the Nexus following the third law, until they can get through to Frisk's conscience (like Frisk did when they were all in Asriel's body).