Gaster taught Sans the magic of creation, and Sans used the rest of the Determination to create Papyrus, Undyne, Asriel, Toriel, Asgore, etc. They lived together for a long time.

One day, Toriel and Asgore adopt a human named June. Papyrus forms a "friendship" with June, but Sans and Gaster become suspicious of it: it weren't them who created her. Gaster finds June and wants to have a long talk with her, but June injures Gaster and runs out of the room, revealing her true colors—Jealousy. Her goal is to absorb the soul of Wistfulness and strengthen herself. She kills Asgore and Toriel and takes control of Papyrus. Gaster is seriously injured by June, and Sans arrives in a hurry. He goes on a big fight with June.

Mettaton 2.0 arrives and helps, but dies of power failure. June absorbs the energy and transforms into a second form, Disappointment. Sans was soon overwhelmed. Everyone realizes that only Sans can defeat June, so they volunteer to give their strength. Sans finally absorbed this powerful energy and became the complete form, Desire. Another big fight.

Sans and June soon evenly matched, then June makes a big move, but Sans blocks. Sans then released a big move, confronting June. Sans felt the energy in his body quickly fade and decided to give it a go. He did his best to blow June to pieces, attacking again, stabbing June's soul into pieces. June is dead.

The soul in Sans's body had been exhausted, leaving only a fragile soul, and Sans used it to create Flowey. It was impossible to resurrect his friends, so he moved on with guilt.

So, a skeleton and a golden flower , went on the way of finding the materials for the soul...