"A long time ago, a human fell into the RUINS. Injured by its fall, the human called out for help. ASRIEL, the king's son, heard the human's call. He brought the human back to the castle. Over time, ASRIEL and the human became like siblings. The King and Queen treated the human child as their own. The underground was full of hope. The human said that they only needed to be patient enough to gain enough human souls to break the barrier. So over time, six humans fell down and their souls were taken."
Over 10 years after Charas fall another human - a young girl called FRISK - falls into the RUINS. Lead by a vague remembrance, she passes the RUINS and meets the two brothers SANS and PAPYRUS. Soon they realized that FRISK is lacking something important, and another yet strangely familiar journey begins...
This AU is the result of different dimensions (not many, 2-3) and timelines melting into one, being forced through determination.
As a result, you have a world where Asriel and Chara are alive, Monsters are hybrids of their original form + humans (so they are not human!) and so on. The world, though it first seems to be completely functional, has its problems and glitches. The plot starts with a 18 year old frisk falling down into underground.