Before the events of Undertale, Asriel found the fallen human, who was the first human to fall into the Underground. They became his adoptive sibling and best friend. When the human died, their last wish was to see the Golden Flowers of their village, on the Surface. Asriel absorbed their SOUL, and the human carried their own body across the barrier to their home village. Asriel clarifies that he and his sibling shared control of their body[3] after their SOULs had combined; he did not pick up the human's body, as the power was shared between the two, Chara picked up the deceased body.[4]
The villagers assumed that the Asriel/Human fusion had killed the child, and they attacked. The Asriel/Human fusion, lethally wounded, and still refusing to fight back or kill six humans, the original plan, returned to the Underground and died. Once deceased, Asriel's dust spread across the garden, leaving his essence on it.