"Mwommy Eirin! Mwommy Eirin!"
A family of yukkuris bounced around wildly as she entered. A bag of food in hand, a warm smile on her face. They had been living there for weeks now, so they all considered her as their mommy, despite their biological mother being in the room as well.
"Hello, Reimu! I see you're taking care of your babies. Do you want some more food?"
As if that needed to be asked, "Yugu! Put it down easy!"
Still beaming a smile, Eirin poured out a bag of yukkuri food into a large bowl in the corner. There was enough room for the small family to all assemble around the bowl, but they spared no time in claiming their place. A quick headcount(rimshot king) revealed the family was growing very quickly- three more than last week. Several of the young yukkureimus had already grown ribbons, signifying adulthood.
Eirin silently grabbed one of the new teenagers and left the room, closing the door behind her. The creature still had some crumbs around it's mouth, and seemed somewhat stunned that it was being kidnapped in the middle of eating. Eirin covered it's eyes as she whisked it past the numerous glass terrariums and observation windows that filled her laboratory.
"Wow! It got dark all of a sudden! Where are you, mommy Eirin?"
"I'm here, little one." She was in no real hurry to get to the end of the hallway, but a panicking yukkuri was harder to work with. "Stay calm for me, okay?"
There at last. The door was already open, she had just left to get a subject. The room was well-lit, but there was not much to see. In the middle of the room was a square metal table, with a small trolley full of tools and a foreign black box sitting on the edge.
"Sit here and try not to move, okay?" She had set it down on the table, "This might hurt a bit, but stay with me."
"Mommy Eirin, what are you doing? What do you mean it will hurt? I want to take it easy!" It wanted to protest, but it was obedient to Eirin and didn't move.
She had already rigged up the black box with a pair of strange two-sided cables with clamps on each end. A "car battery" and "jumper cables" as Rinnosuke had said, and it was within his ability to tell what they were for;
"Mommy, those look sharp. What are you doing with them? I want to t-UuuUUUUuuuUuuUUUU!"
Eirin already had her notepad out and was scribbling like mad. Yukkureimu had gone completely rigid. Tiny bolts of electricity wove around the clamps and arced to the table. After twelve seconds, the interesting parts started. Yukkureimu seemed to be slowly melting.
Another five seconds, yukkureimu stopped trying to scream. Another three, her eyes melted. Another two, her hair was gone. Faster than it started, everything stopped. Eirin carefully leaned in to examine what she presumed to be a body.
It certainly seemed dead. The accessories were more or less intact, unlike melting in water. The only thing that indicated it was alive was a low vibration, almost a ripple across it's now fluid body. Eirin quietly pulled off of the jumper cables and poked it with a scalpel. Much to her surprise, the scalpel was sucked out of her hand!
It began to slowly pull itself back together and reform. The skin began to turn transparent, the bean paste within it was gone but the scalpel was stuck within it, suspended in some gel. The accessories were likewise sucked into the center. The creature, when it began to calm down, was no larger than a computer monitor. It spent the next few days in a glass box, under observation. Eirin recorded her findings, few as they were, before dumping the horror into the sewers like she did with all of the experiments in the end.
She didn't know what horror she had unleashed upon the underdark. She didn't think it would reproduce or grow. It took hundreds of years for that one experiment to produce enough offspring and cause enough trouble to gain a name.
The gelatinous cube.