(Part 2 was requested, so I'm gonna make an attempt to make this story better. Hope you enjoy!)

~Time-skip~ (3 years later)

Nightmare's P.O.V.

"(Y/N)." I called, waiting for my loyal hound to come into my throne room. She came trotting in and sat in an attentive state (image below); waiting for me to give her a command. "Lay down." She does as she was told and lies down beside my throne.

She has been my loyal hound for three years now. She has grown to be quite a bit bigger than a fully grown German Shepherd. Her strength is quite terrifying as well. She has been able to tear apart any attack Ink, or any of the other Star Sanses, could throw at her. She's even dusted an entire AU on her own, though she was under my watch when she did. (Y/N) is such a loyal and sweet hellhound.

"Good Girl." I then pet her head, careful not to mess with her horns. They are very sensitive and she doesn't like having them messed with. Horror found that out the hard way. I then chuckle at the memory.

~Flash back~

(Y/N) was laying down lazily and seemed quite bored. I was bored as well, having nothing to do but wait for someone to report to me. I then had my wish come true and Horror came in. "We finished our job, so what now?" (Y/N) got up and rubbed herself against him to get his attention. He pet her horns, not paying attention to her at all. She then growled and moved her head a bit, but Horror paid no attention to her. I answered him and told him that he could go.

He then continued petting (Y/N) on her horn and she bit him; Hard. "Ow!! What the-!?!?" He didn't have time to finish his sentence because (Y/N) had kicked him down with her back leg. She then trotted over to me and sat in my lap. "Why you little-!!!" I then glared at him, causing him to stop mid sentence. He left so angrily that he punched a wall, but he did it with his injured hand, causing him to yell out in pain once more. I was silently chuckling whilst (Y/N) was watching him leave.

~End of Flash back~

Speaking of Horror, here he comes now. "You needed me?" I nodded and checked what AU he needed to destroy next. "I'll let you choose. You can either destroy Undertail, or Cyndertale." Horror smiled his crooked smile and chuckled. "I'll take both." I gave him a dark look. "I said either. One is for you, and the other is for (Y/N). Make your choice, or I'll pick for you." Horror rolled his eye light, but picked Cyndertale. "You can leave in two hours. Dismissed." Horror then walks out, but avoids (Y/N) when she walked past him.

"It looks like you and me are going to have some fun destroying this AU, don't you think?" (Y/N) barked and started wagging her tail. "Alright, Alright. We'll leave in a moment. First we have to get your leash." (Y/N) whined in disapproval. "I'm only doing it to make sure you don't jump through the wrong portal." She stopped her whining and let me put her (F/C) leash on her. She seemed happy with this leash. Her last leash was torn apart. It's was (L/F/C) and seemed to make her uncomfortable.

I then opened a portal and walked through with (Y/N) following behind me. I then looked around and made sure we were in the correct AU. "Perfect." I then kneeled down to (Y/N) and held her collar, but not pulling it. "Are you ready to have some fun?" She started to bark in excitement. "Good girl. Ready?" I unclip her leash from her collar, but keep my grip on her collar. She starts to squirm around, but she doesn't get loose. "3, 2, 1. Go get them girl." She then spring off towards Snowdin. I then teleport around a follow her.

Narrator's P.O.V.

(Y/N) was sprinting into town looking for any monsters to tear apart. She was trained by Nightmare, so she's got a knack for killing things. "Wow! Look at it go! Wonder what it's looking for?" A bunny monster said aloud, causing (Y/N) to snap her attention in that direction. She the started to run over to the monster, but instead of slowing down she jumped on top of the bunny and bit it's neck. As the monster turned to dust (Y/N) then went off for another monster to kill. (Y/N)'s snout was now powdered in dust, but almost impossible to notice because of the bone.

~Time skip to Sans~

(Right now Undertail Sans is the only one alive, much like a genocide run. Except Asgore is dead before Sans is.)

Undertail Sans wasn't very happy with this dog killing so many monsters, so he decided to finally come and kill it. "Where are you, ya mutt? I've got a bone to pick with you!" (Y/N) just came out walking, but was now coated in monster dust. She was growling at Sans. His magic was stronger than the other monsters in this AU, so she knew she couldn't play cute. "Come on ya filthy flea bag." (Y/N) then ran into the woods. "Get back here!" Sans yelled, quickly getting peeved off with this 'dog'. (Y/N) was able to slowly sneak around him by staying in the shadows of the woods. She then jumped out from behind him and attempted to bite his neck, but he was quick to react and shot a bone at her. She surprisingly, only surprising Undertail Sans, caught the bone. She then broke it with her jaws, turning it to dust.

"What are you?" Nightmare then came behind Sans and shot his tentacle through his back. "She's my Hellhound." Undertail Sans then turned to dust and (Y/N) barked in glee. "Good job (Y/N)." Nightmare then started to pet her dust covered head and hugged her neck. "You're such a good girl. When we get home, I've got a treat for you." (Y/N) barked and had her tail wagging like crazy. Her treats were usually raw meat, but mainly raw steak. She LOVES steak. She was going to be a very happy hellhound when she got home.

(I hope you enjoyed! Sorry if it turned out horrible. I tried my best with this idea. Have a great day!)