"The void?"

Sans stretching up on his feet, peering down onto the small plaster tray in front of him.

"What does the void do?" Sans asked, his childlike voice rising in volume.

A man, tall in height, with a face Sans couldn't remember, stepped around the table. "In small doses, it increases a persons strength. It's like a power boost, really. Monsters might be able to break through the barrier with it. However... it has some drawbacks. On all of the animals I've tested, their souls don't have enough hate within them to properly respond to the void."

"What does that mean?"

"It means the void is directly born hatred. If a soul doesn't have enough hatred, the void becomes worthless, and instead it just consumes their soul and destroys them. I found a few ways to bypass this, but most of them are rather useless. Except for one, but... you can see how it turned out."

A hand with a hole gestured to a wall, where a small glass pane lay between their room and the next. Behind the door, a deer gurgled, it's teeth rotting and one eye falling out.

"Ew, what happened to that?" Sans asked, recoiling in disgust.

"I found that the void responds to one specific vegetation," The man explained, face lost in Sans' memories. "Fungus. It you combine it with specific fungus, the natural growth patterns of the fungus acts as the hatred catalyst needed for power." He stood in front of the glass, hands folding behind his back. "However, this seems to come with a toll. Hatred, within itself, is the disdain for any over living form. As a result, combing a fungus that feeds off of mammals and reproduces by spreading spores is evolved into a mindless cannibal who has the natural desire to consume anything living of its kind. The void doesn't power the hatred of the animal, it powers the fungus, the thing that contains the hatred, and simply uses the body as a host, just like a regular fungus would. It's better I chose to use animals instead of monsters, or else the fungus might have spread."

"Are you saying fungus is angry?" Sans asked.

"No, fungus cannot feel emotions. The natural feeding of this species is simply one that matches the feeling of hatred, which the void can connect to and power up. While the body becomes weak and sluggish, the fungus takes over the mind and seeks others to spread the disease, or in this instance, hatred. The body is nothing but a host, and I've observed that after the fungus has been 'starved', it dies off and returns all remaining body functions to the host. Of course, this is for an animal, much less a simple fungus, but I cannot properly test on monsters without the risk of exposure. We're contained Underground, it's far too risky to release a fungus that can spread upon a trapped population, especially when I don't know it's effects on a fully sentient and complex organism. If I could find a host that replicated the hatred needed for the void to grow, that would be amazing, but monsters by biology are creatures who need love. If I were to use a subject, I'd need a human, but alas, there are none down here. We have no humans that have yet to fall, and I'm unsure how long it would take."

"Where did you get the void from?" Sans asked.

"My human colleagues and I, who studied alongside one another before the war," The man hummed, "But it should take them longer, especially since I have most of the notes. Sans, I'm going to have to burn my research."

"But *&#^*, you love your job!"

"I do, but my research of the void will become a hazard. Imagine what desperate people would do with my research? If someone managed to connect something to the void like I did, they'd be able to create the perfect fighting machine to kill everyone without hassle. Or, if that person doesn't have enough hatred, would create a disease so terrible that it could wipe out an entire population. The humans hopefully don't have enough research, at least to last until we escape to the surface. I'll continue my research until I find a human suitable, but if I don't, the moment we get to the surface I will destroy all of my research and theirs if I get there in time."

"What if you can't?"

"Well, most likely we'll escape to a normal surface. Unless they manage to find enough research and a good human specimen to experiment on, I doubt they'll manage to get very far. If anything they'll have a few mindless specimen, and given a year or less, would recover most of their abilities. And as long as the specimen are contained with proper procedures, they should be fine. I'm far more worried about humans finding enough of hatred filled souls to create a small army of void filled creatures. Humans are naturally strong, but filling one with the void will lead a person to hone in on their hatred and become stronger because of it. That's a far more concerning opponent than a few mindless fungus creatures who could be easily held and dealt with."

"If they're made of hatred, does that mean they can't love?" Sans asked.

"Oh, Sans, no. The void only builds off of immense hatred, or fungus that closely resembles hatred, to power it up. Just because the hatred is amplified doesn't mean the person cannot feel love, it just means it's more rare. If they did fall in love, it would be a very close connection, one of souls itself. It's not impossible, they are more lenient towards hatred, as it is the source of their power."

"Does that mean if we p*ss them off lots they'll become even stronger?"

"Language!" A hand lightly flicked against Sans' skull. "But that is the best way to summarize it. The more hatred you give them, the more their power would grow. So it'd make sense to make them angry to further their power. I wouldn't recommend that, as you'd have a genetically enhanced human, who's stronger and faster than ever before, mad at you. Not a good idea."

Sans jolted awake against the cold pillow, breathing harshly through his naval cavity. That was the first time he had a longer memory, it seemed his memories were starting to pick up.

"Hm?" Jane asked groggily, shifting under her blanket. "You want more cuddles?"

"I don... don't trussssstttt Friskkkkkk," Sans gurgled.

That caught Jane's attention. She sat up further, her hair bunching over her shoulder with the movement.

"What?" She asked.

"Frisskkkkk haggggg... had yelllooowwwww eyyyeeessshhh..." Sans said.

"Yeah?" Jane asked.

Sans shook his head, looking over. "Noooo, before. Beforeeeeee... frommmmm memerhiesss... so whyyyyyy... do theyyyygghhhh haveeee reddd eyessss now?"

《》《》《》

Okay, for explanation, BOTD has it's zombies made only from fungus. However, in this story, it's a fungus enhanced by the void, or if you want hate juice, that lives in the host and tries to spread, thus creating zombies. It combines both Jane's backstory and Undertale with BOTD so yeah, going this way. After all, how the heck would zombies be created without a genetically enhanced fungus?