The lore behind the last three songs of this playlist
At the end of the Asriel battle in the genocide run, Flowey will appear to absorb Asriel's soul before it shatters. Even if he is decimated after killing Ezraan.
On that note, Chara will regain his body after absorbing his brother's soul.
On the first time, he'll say, "I knew it was a mistake to make that deal with you."
If you decide to do another genocide after killing him, a few things will be different.
Hera will make snail pie
Napstablook will actively hunt you down in the Ruins, but will avoid you after you kill Hera
Papyrus uses a shock button in the hand instead of a whoopee cushion in the hand
Chara will say, before his fight, "You sicko! Why are you doing this?"
Fifth genocide in a row is hard mode. Before Chara fights you, he'll say, "You sadistic b###h! Why do you keep doing this!?"
After the fifth genocide, you choose whether to do genocide before selecting a name. If genocide is chosen, Lv begins carrying over because the difficulty skyrockets with each genocide.
On the tenth Genocide in a row, Chara will say, "YOU! I HATE YOU! YOU KEEP PUTTING THIS WORLD THROUGH THE SAME HELL OVER AND OVER! WHY!? WHAT WILL THIS ACCOMPLISH!?"