Shattered Timelines - An Undertale AU

This is an Undertale AU that I thought of myself that adds a couple of new monsters into the mix. When making this AU, I was thinking of a way for it to be different and I thought to myself, "What if we followed a character other than Frisk/What if Frisk died?" So I combined those to ideas and put them into an AU.
==Meet Vin and Mattius==
Sometime ago, two wolf monster moved to Snowdin Town, a Royal Guard member and his little brother, their parents killed in the War for the Surface. Mattius is the older brother, he is the captain of the Canine Unit in Snowdin and is just as skilled a fighter as his Captain, Undyne. He is a Grey Wolf-like monster with a scar on one of his ears, his weapon of choice is his Great Sword which he named "Human's Bane". Vin is the little brother and the one that this story follows. As soon as Vin and Mattius moved to Snowdin from the Capital, Vin became friends with the one known as Monster Kid. Vin and Mattius's lives were normal, until the Seventh Fallen Child appeared.
==The Adventure to Save All Monsters==
Vin woke up that day expecting everything to be fine, Mattius had already left for his patrol and all was quiet, a normal day in Snowdin. At least, until someone knocked on his front door. Vin went to go answer the door and stand outside was a hyped looking Monster Kid. (I am gonna have the characters talk script style.)
Monster Kid: Yo dude, you won't believe this! There's a human in the Underground!
Vin: Huh? Are you sure?
Monster Kid: Yeah, follow me! I know just who to ask!
Monster Kid runs off and Vin follows him. The two run to the house of the Skeleton Brothers, Sans and Papyrus. Monster Kid knocks on the front door and a couple minutes later it is opened by the tall skeleton, Papyrus.
Papyrus: OH, WHY HELLO SMALL ONES. WHAT DO YOU NEED OF ME, THE GREAT PAPYRUS? 
Monster Kid: Tell Vin what you just told me Papyrus!
Papyrus: ALRIGHT, IF YOU INSIST! SO, IT STARTED IN SNOWDIN FOREST... 
Papyrus tells Vin about how he and Sans met the human, tried to trap the human, fight the human, then befriend the human. Vin's eyes widen at the befriending part.
Vin: But I thought all humans were bad, how did you befriend them Mr. Papyrus?
Papyrus: WELL, AFTER THE HUMAN DEFEATED ME, THEY SAID, "LETS BE FRIENDS!" SO THEN ME AND THEM HUNG OUT FOR A BIT AND CERTAIN... FEELINGS... WERE REVEALED. THEN THE HUMAN LEFT TO GO TO THE CAPITAL SO THAT THEY COULD GO BACK TO THE SURFACE. 
Monster Kid: If they just left, then that means that maybe they're still in Waterfall, come on dude lets go! We might be able to see Undyne too!
Vin and Monster Kid head to Waterfall to search for the human that Papyrus talked about. After making their way through shallow waters and marshes, they reach a bridge with a sudden drop.
Vin: Woah! That's a long way down.
Monster Kid: I think that leads down into the Garbage Dump dude.
Vin: Well how are we gonna get down there-
Monster Kid jumps off of the bridge and Vin looks down in horror.
Vin: -WITHOUT BREAKING OUR NECKS!
A splash is heard, then silence.
Vin: Hey, are you okay down there?
There's no response for a couple of minutes, then someone can be heard coughing.
Monster Kid: Yo, I'm okay dude.
Vin: *Thinks* That's a relief. I'm gonna jump down next!
Vin does as he says and plummets into the darkness, when he suddenly lands on something soft. Vin has landed on a patch of Golden Flowers.
Monster Kid: Duuuude, you go so lucky with your landing, all I got was a mouthful of water...
Vin: What are flowers doing growing down here? There's no sunlight.
Monster Kid: Don't know dude, come on lets keep going!
Vin and Monster Kid continue their trek through the dump and head to the end of Waterfall. They are stopped at a bridge by RG 01 and RG 02.
RG 01: Like, sorry dude, but we're not allowed to let anyone through, it's Captain Undyne's orders.
Vin: Can you make an exception for a Royal Guard in training.
RG 02: ...Sure.
RG 01: Are you sure bro?
RG 02: *Nods*
Vin and Monster Kid continue across the bridge and see Undyne and Mattius talking, there's someone lying on the ground, a puddle of red under their body. A spear is sticking out of their chest.
Monster Kid: Yoooooo...
Vin: Are... are they...?
Vin gets closer to the body of the Human. A red heart-shaped object is floating above it, the Human's SOUL. Vin touches the SOUL and the SOUL is absorbed into Vin's body. Vin now sees a new thing at the end of the corridor, a star-shaped object.
WIP