In Doomking's AU, Duel Monsters, every monster is based of a yugioh card or architype, for example the monsters in the ruins, asgore and asriel are salamangreats, the snowdin monsters are zombie horde, waterfall is orcust and hotland is any meta relevent deck.

The war happens after the humans find out that the monsters, mainly Exodia (Gaster) and Crystron Alqifibrax (Alphys) have been experimenting on humans on the overworld to obtain determination and improve their ability to absorb souls, at the time the war started they had a weapon that could only put their soul into a human's soul. It didn't do much for the monster except it let them spy on them and the things around them and to tease and play mind games with the human until they figure out how to supress the monster but it gave the human the magic powers of that monster and the ability to absorb other human souls by storing them in the monster's soul...

but if the monster was to ever get out after doing that, only the monster would have the souls absorbed.

It is a prototype of a weapon that could make monster's absorb human souls while skipping the fight. Sans decided to take a prototype and the war began. While at war, Sans faced a human named Harry. After defeating sans Harry spared sans and ran off to safety. After that Undyne attacks and nearly kills Harry but sans comes to the rescue and uses the weapon fusing their souls causing a situation like Chara in undertale.

The war ends and the monsters are sent into the underground, except for sans. For three years sans stays in Harry's body undetected gaining information about humans and learning the powers of their fused soul. Harry with his new powers is called a freak and is discriminated causing him to lash out at humanity. He attacks, kills and absorbs the souls of three humans. He is on the run from the police and special force departments when sans appears to him for the first time. He leads Harry to the underground where the events of undertale happen but with two differences, Frisk moto and Yami Chara are in the underground living with toriel, after their power of determination was taken away by Harry, and sans never came back to papyrus after the war.

Harry does a pacifist route first and battles frisk and toriel, in this battle he learns about his determination and about sans' powers. As he enters snowdin instead of sans appearing he is sent straight into a battle with Jack o' Papyrus. After the war papyrus assumes that the humans killed sans so every human that has come here except for frisk/chara. This battle is alot like undynes fight where you have to keep running away and going back into the battle. Eventually sans tells you how to teleport which back fires teleporting them into papyrus' house in an empty room to the right of the house. As they leave for waterfalls Papyrus catches up to them and fights them once again. Eventually papyrus realises that Harry and sans have been fused and reluctantly walks off.

In the Undyne fight, Harry remembers how undyne tore off his sister's arm and how she nearly killed him. He would've killed undyne if it wasn't for her determination. As the fight continues sans continuously tries to get through, past the mental wall he put infront of him, to Harry but it's no use... until

Eventually Harry tires out and the wall disappears and sans who is pushing against the wall over shoots and accidently takes over Harry's body causing Harry to become to "ghost" inhabiting the body (like some of the fans invision the chara and frisk during the sans fight). Undyne cools down and the continue on.

In the judgement hall Papyrus takes the role of arbiter and he and his brother have a talk. The rest of the game goes as normal with Salamangreat Heatgore (agsore) and Spinny (flowey) battle going as normal. Then the actual pacifist route happens.

Sans prevents Harry from going into the underground lab (in fear that he might find out about the weapon) and instead they get undyne to come over. Undyne fixes things, Dark Magician (Asriel) battle happens as normal and the monsters are let out of the underground, unfortunately.

As soon as the monsters escape sans traps Harry in his body and all hell breaks loose. The weapon that the monsters were using had been finished and the monsters were using it to steal human souls (killing them) and becoming even more powerful. The weapon has many settings; fusion, syncro, xyz and link. Each with their own special power. From within sans' body Harry enraged by what they were doing peforms a true reset where Harry traps and tortures sans inside his body and performs a genocide run.

The differences in this battle are, Toriel absorbs frisk/chara's soul after Harry kills them, papyrus escapes the battle in snowdin, undyne was the same, mettaton and muffet team up and finally papyrus tried to use a prototype of the weapon against Harry but fails. Through out the battle Sans tries to turn off Harry's bone magic but is eventually knocked out by the pain Harrt inflicts on him.

After killing papyrus, asgore and flowey. Harry walks towards the end of the barrier about to unlock it when sans escapes his body using the six human souls. With the power of the 8 human souls (counting frisk and chara) Sans faces off against Harry to end the series.

I hope you liked it and if you made it this far comment "where is kuriboh" XD thanks so much and happy one year aniversary.