Epic of Caterpillar

The Necrotic Death Lords quickly began to create their facility made entirely out of Necromancy. Inside of their Necrotic Souls there were spaces similar to Divine Realms, but they were lands of dead and decay named Nether Realms. Inside of them these powerful Undead grew unique Necrotic Materials for the usage of their techniques and spells. They all utilized thousands of such harvested materials from the interior of their Nether Realms and began to quickly conjure the creation of this new facility.

The powerful facility or well, a Building-like Undead was actually something close to the Netherworld Plane's version of Formations. Because in this mysterious Plane the concept of magic as we know it was not useful, nor there was Divine Energy to make Formations. The people of this world had designed their own unique ways of making formation-like structures, through Necromancy!

In a world filled with Undead to the brim the natural way of making things up was through the power of summoning, creating, and binding the Undead together, Necromancy. Such a power had never been developed as much than in the Netherworld Plane, where Necromancy was like the brad and butter of everybody who wanted to learn "magic-like spells".

Necromancy accompanied with the power of Death, Phantom, Poison, and Darkness were all the elements that these entities could wield. They combined it through Necrotic Techniques that used their Necrotic Energy and also their Necromancy, their version of magic.

Through it, they used the materials in their own Plane and also in their Nether Realms, creating Undead out of them, molding them to do many different tasks, attacks, powers, and abilities, fusing them to create new things, or even forming such a massive and bizarre Undead monster that it could be used as a factory that absorbed demonic energy and then repurposed it into usable energy for them to slowly form a Demonic Core.

In the Netherworld Plane they had adapted to such abilities over eons, mastering them every single million of years into an even more perfected form to the point that this was the first Realm to have awakened the powerful Dao of Necromancy capable of aiding them in their creation of Undead, Curses, and Devilish Incantations, amongst many things related to these things.

This world was a world where the Undead used each other as resources and items to fulfill their own selfish goals, although many would think a world of Undead would be stagnating and silent, cold, and boring, this world was in fact filled with wars, battles, and constant events as the many Undead of this world fought for supremacy and sustenance. Undead were not without the need to eat, as they required to devour each other souls to sustain their own selves for longer, without absorbing enough souls and necrotic energy they would stagnate, grow weaker, and end up as little and pesky undead that would be easily destroyed with the wave of a hand.

With such a fact in this world, the Necrotic Death Lords were those Undead that had reached the pinnacle in terms of the "mortal hood" strength aspect of the Undead, those that surpassed the levels of Necrotic Masters would become Necrotic Death Lords, equivalents of gods to mortals.

And those such as them had already reached a power comparable to the Supreme Gods if not higher, the thing was, this plane was immense to the point that there were many Supreme God-level entities living within, now that they were being brought outside into Genesis their power weakened due to their lack of an Origin for this Universe, but even then, using their "anchors" such as Hel and this Archdemon Avatar, they were able to exist within these places and unleash part of their original abilities and powers even if they shouldn't be able to!

Now the Necrotic Death Lords were able to unleash this powerful Undead entity which they had constructed using Necrotic Materials and many souls and corpses of Undead Beasts they had hunted and slain, and fused them together into this monstrosity, capable of absorbing the energy of hell and transform it!

They wanted to transform this energy into something less "potent" and more "mild" which would allow them to better assimilate it and fuse it with the power of their necrotic energy, making up the power they required to make their own demonic cores, so they would be able to easily wield the power of the demons and hence acquire the full origin of this place, being able to unleash all their original power as well!

The gloomy and dark streams of Dao Essence coming out of them from their Necromancy Dao resonated across their bodies and souls, coming out like large streams of pure energy which reached their Undead, boosting its power through the roof and making its effects even better. The work it was doing quickly hastened as the power of the Demonic Energy was absorbed successfully into their bodies through their connection with this Undead, slowly yet steadily materializing into demonic cores, red and orange-colored crystals within the interior of their ribcages.

Just a few more minutes, perhaps an hour… and they might be actually able to accomplish the thing their people had been trying to accomplish for eons now! Right in the palm of their skeletal claws was the next step towards their expansion outside their own Plane… The Gates of Hell were now a simple battery for even more energy they could get, and more powers they could find…

However, as the Necrotic Death Lords absorbed this energy and made it theirs, Kireina had emerged with Rimuru at her side, and a group composed of some of her family and her allies, ready to fight off these invaders of Hell by the demonic command of the Prince of Greed, Mammon!

"Hey, why don't you share some of that meal with us?" she asked mischievously.

After her mischievous words, the Necrotic Death Lords looked at her with utter fear! In this moment… they were actually rather vulnerable!