Putting a paw to his chin and glancing off to the side, Asriel tried to piece together any memories he had of the strange monster man. There were plenty of happy moments with two jovial skeleton brothers, but none of them came remotely close to matching Luna's description. But like a putrid lump of coal in a pile of shiny gemstones, one recollection stood out for all the wrong reasons.

"It all happened a while after the barrier broke, one day when Frisk, Chara, and I were playing around in Waterfall. We stumbled upon an open metal door that we had never seen before. We looked inside, but it was too dark to see anything."

"We started half-joking around, saying that Mettaton was probably filming a horror movie or something. Chara dared me to go inside, and when I did, the air got really cold around me. It was also really dusty. I walked around a little and heard a scream that made me jump out of my skin, and when I backed away, I bumped into a tall monster in a black robe. The monster looked at me with his horrible, cracked white face and said..."

"The darkness keeps growing...the shadows cutting deeper...dark...darker...yet darker..."

"His voice wasn't loud, but it hurt my ears like a piece of glass scraping against a chalkboard. Then, he reached out and tried to touch my face. Before he could, I ran screaming out the door and into Frisk and Chara. I told them what happened, and we ran back to our parents and told them all about the creepy guy in Waterfall. We all went back to Waterfall, but the metal door was closed and locked. Mom and Dad told us not to go back there again."

The M.B.'s newest commander smiled quietly and gave his witness a hearty pat on the back.

Happily, he announced, "A secret base hidden underground, protected by the one thing that we couldn't find a way around? Clever. Just when we worried that the leads had frozen forever, the M.B. now has a hope of finding Gaster. You all have my sincerest gratitude."

The commander signaled to the carrier, which landed beside him.

"We'll take you back to Earth, but now would be a nice time to say farewell. Even though we're headed to the same place, I'll be leaving for ZY-9 immediately after I search Gaster's house of horrors. It was (mostly) a pleasure meeting all of you, though, and I'll definitely be taking up your offer in the future."

"Wait."

"Your Highness?"

"If this Gaster is as bad as he seems, you're going to need all the help you can get."

"No. I can't thank your son enough for the lead, but the Dreemurrs have given us all the help we'll need. This operation is easily ten times as unsafe as the one you were shown. You two need to stay home with your children until this is all over."

"We are not figureheads, commander. We have a duty to protect our subjects as king and queen; if that includes fighting, we're far from pushovers in that regard."

"Fair enough, but do not underestimate anything you see. Don't forget that Gaster has eluded the full force of the M.B. for years. Few know what he's capable of, so stay close. Safety first, even though the safest place for you all is far, far away from Waterfall. If something goes bump, do not go looking around for it by yourself. If something brushes up against your leg, there's no shame in running screaming out the door again."

No point in trying to talk them out of an operation in their own territory.

"None of this, of course, applies to you three. You'll be home and playing Humans and Monsters or whatever it is humans play with monsters."

"What? But that's not fair! Why do Mom and Dad get to go chase bad guys?"

"Mom, Dad, the M.B. is so awesome! I want to be a cool crime fighter like you guys! How long before I get to help you beat up those mean humans?"

They were like superheroes, and while everyone wanted to be doctors and scientists, I never thought of anything else for myself but being them one day. I'd even been practicing with magic, too, so I raised a claw and created a glowing, white star at my fingertip. Mom was impressed; she clapped her paws together and spoke in sweet tones while Dad playfully patted my head.

Luna's breathing faltered and his gaze un-focused for a moment, but he composed himself and responded, "It's too dangerous. Non-negotiable; you're staying out of Waterfall until your parents tell you otherwise."

Everyone, feeling varying levels of indignance, boarded the waiting destroyer, which departed from the asteroid, activated its wormhole generator, and headed straight for Mount Ebott. Jumping out of a wormhole, the vessel emerged above the ruins of the human village and landed near the cave entrance to the Underground.

After a few hours of sneaking and ducking around the locals, Luna, six moderately armored agents, and two Dreemurrs gathered in front of the mysterious door in Waterfall. His voice interrupted the sound of trickling water.

"Here we are. If any of you would like to reconsider, this is your last chance. We have no information about the contents of this room; it could be empty and we'll have come here for nothing, or it could be a trap with some ungodly surprise in wait."

Nobody reconsidered.

"Very well then. Everyone ready?"

Luna crouched down and inspected the grey, steely doorknob from a lot of different angles; apparently satisfied, he gingerly reached for it with his paw. Before he could turn it, the entire door fell over by itself with a thud.

"Back! Everyone back!"

Nothing happened, which only further unnerved everyone. The pitch-black rectangle Luna was shouting at seemed infinite, as though a step inside would plunge them into a cold void for all eternity. Despite being covered in fur, the investigators shivered as the air throughout Waterfall seemed to plunge about twenty degrees. Asriel's parents became pale as a sense of dread loomed over the group. Luna took the lead and the agents followed, treading quietly, and groping around the walls for a power socket.

"This one's safe. Turning on the lights now," reported one agent.

A single fluorescent lamp in the middle of the ceiling buzzed to life, illuminating the lab with a weakly insufficient greenish hue. Even though the bulb couldn't reach into the dim corners, it became obvious that behind the metal door was a huge lab, walls lined with terrible instruments and machines Gaster undoubtedly used for his twisted experiments. One machine was lined with bloodstained spikes. Another had two buzz saws attached. A third was a dentist's chair with mechanized claws hanging overhead. The lab must have measured at least twenty feet on all sides, but every cc of air reeked of dust and decay. The Dreemurrs took one guess where the dust came from and took on the same shade of color as the ambient light.

The commander instructed, "Fan out and look for Gaster, but keep eyes on at least one other agent. It's pretty obvious that we're not welcome here, and I don't want anything to happen to our 'guests.'"

The agents donned four-fingered gloves and began to search the lab, peering around each and every machine. Every so often, one of their ears would twitch slightly, as if to pick up an almost imperceptible whisper. No matter how many times they turned around, the nagging, burning sensation of being watched never fully went away. Eventually, they reached the end of the room, but despite every rattled nerve shouting otherwise, there was no sign of Gaster.

"No one is here, sir."

"Check for clues. Gaster may have hideouts outside of this one. We've come too far to lose sight of this target again. If you've opened a cabinet, listen for any secret mechanisms. If you've touched a bookshelf, check each and every page for any passwords. If you've turned over a stone, turn it over once more."

Some of the agents began to sweep Gaster's disturbing furniture choices with a black light, hoping to uncover telltale traces of the mastermind. Others rummaged through desks and cabinets in search of secret documents and possible leads. Suddenly, the overhead lamp sparked and failed.

"Hey, what the— "

"Who touched the switch?"

"Who just bumped my leg?"

"Help! Something's got me!"

The light began to glow a toxic green again, amidst a flurry of voices from agents.

"Is everyone okay?"

"No signs of explosives or chemicals, sir! We kept an eye out for booby traps and tripwires and didn't find any."

"Sir, we're not missing one head…actually, we're up two!"

"Excuse me?"

One voice came from an unlit corner behind the commander.

"Come to find your parents, little monster?"

Luna turned around.

The black robe melting into the ground as if it was a putrid, oily spring.

The two-forked, cracked skull frozen into a malicious grin.

The hand holding a scalpel against Asriel's neck.

Asriel himself was tied to a blood-soaked gurney.

"Sir, he's got a hostage!" an agent said worriedly.

The commander raised a paw, warning everybody to keep back.