Of course it wasn't long before Asriel, wracked with grief, fear, and dread, transformed back into the small golden flower he once was. It was not his size that despaired him. It was his heart.
In this form he could not feel emotions such as love or grief. So what was to stop him from destroying everyone in his path?
Terrified he would ruin Frisk's only chance at happiness, he ran into the ruins to hide himself. To his horror, Frisk found him standing by the bed of flowers the determined hero woke up in.
Frisk comforted him and accepted his apology without hesitation. Grateful, Asriel bid goodbye and Frisk returned to their new family.
That night, nearing his transformation, Asriel reached Toriel's old home. He opened a door that said "Room Under Renovation" in a final attempt to hide. What he found surprised him.
All that was in the room was a flower pot that shone like a flame.
That was the last anyone heard of Asriel.
Meanwhile, Toriel, Asgore, Frisk, and their extended family, escaped the ruins to the Human World. The kingdom was free.
Frisk led the monsters to a village where the news was spread. Before long the entire World knew of the Monsters' return.
The Humans were reluctant to let them return, since life had been well without them.
They built them a civilization apart from the majority of humanity. The Monsters were quite isolated. They still tried their hardest to be normal, but the humans never acknowledged them. It was as if the Monsters were banished, but this time to the Human World, alone.
But the Monster's were grateful nontheless. They had fresh air, a blue sky, and stars at night. No matter how isolated from the humans they were, they were better than they used to be.