"This time I might just disappear."
– Asriel while being a ghost
Ghost!Asriel is the skeletal resident of an unknown place and former biological child of Toriel and Asgore, and the adopted brother of the late fallen human. His goals are to seek revenge to kill the human, kill whoever came to the Underground, and reunite with his family.
About the AU
What if Flowey never existed in the Undertale series? So here's the story of how Asriel become a ghost:
After Asriel returned the dead fallen human to the village after being attacked, he was lethally wounded, returned to the Underground and died. But his dust doesn't spread across the garden, instead, Asriel's dust went into the jar that Asgore was holding. Asgore constantly carries Asriel's dust around in a glass jar with Toriel.
Later, Toriel and Asgore went into Alphys' Lab, they told her to revive Asriel with Determination to make a happy family again. So, Alphys takes the jar and injects determination into the dust that had bloomed in the jar just before trying to make Asriel return to his family again. Initial experiments on the dust vessel proved successful. Once the vessel was replanted, Asriel was brought back to life, and Alphys returned him to his family. The Dreemurr Family were all live happily ever after.
But something went wrong, Asriel is not feeling well. Toriel told him if he's okay. But suddenly, it reveals that Asriel was melting, and Toriel was shocked and ran to Asgore and tell him that Asriel's body can't handle with too much determination, the magic that composed them lost cohesion, and he completely melted into strange and horrifying goat monster. And it was too late for them. Toriel and Asgore was really upset about Alphys injecting too much determination to their son.
As they were not happy about this, they continued to take care of Asriel as an Amalgamate. They eat, they laugh, they play, take care, and LOVE each other. Until that day, Asriel had completely melted into a hopeless, pile of puddles, like how the Bromalgamate melted. And thus, Toriel and Asgore now lost their son and put the puddle into the coffin. Toriel put the Heart Locket into the coffin and Asgore put the first flower next to the Heart Locket. And they say goodbye to Asriel and their hope was lost.
The tragedy of Asriel's deaths led Asgore to declare war on humanity in a fit of anger. He swore to kill any human that fell into the Underground and planned to use their SOULs to break the barrier and destroy all humans. Toriel, disgusted with him, left New Home and returns to the Ruins.
- Underfell Ghost!Asriel: An UF!Asriel in Angel form and Ghost!Asriel are looked combine together. In Underfell universe, Ghost!Asriel now referred as "The Lost Angel, Asriel."
- Underswap Ghost!Monster Kid: Ghost!Monster Kid acts the same as Ghost!Asriel does. He looks like a fusion of Monster Kid in Hyperdeath form and W. D. Monster.
- Ghost!Asriel is based of Lewis, the Paranormal Investigator who was being pushed by Arthur and seeks revenge on him and trying to reunite with Vivi.