The returning trip was calm. The milky fog and the freezing, viscous atmosphere felt thin and soft. With her control over her slime increased in subtlety and mastery, Iris casually strolled through the cave undeterred. The Corruption Power inherited from Vilia gradually merged with Iris, filling her soul with pleasure and knowledge and the mischievous disposition.

As she crystallised her power, she wondered about her strength. She wasn't sure how strong she had become, but she could feel a natural limit approaching. Vilia was a strong Monster Girl, though not as strong as Lenmia, who was at the peak of the Metamorphosis Phase. Iris was becoming stronger but felt weaker than before.

The power of those beings was too high, so high that she felt dizzy when she tried to see their peak. The road was clouded in cryptic mist, and she wasn't sure if it was possible to become that strong. She hated that any of them could flick her and her family into nothing, instantly, effortlessly.

If she were that strong, she would be able to protect her family.

Iris ran across the dark, musty cave up to the ground, scaling the slope, passing through the familiar terrain. The ices and snows gradually vanished, replaced by the cracked rocks and fluorescent plants.

When she reached the normal part of the cave, she immediately noticed the ominous stillness which permeated the cave, its soils, and its earthy fragrance. The sounds of the dews dripping echoed around the circular ceiling, resonating with Iris's footsteps. She condensed her lower body to resemble her human legs, aiming to surprise her sisters with her improvement, but she found no one.

She passed the pool where Dulcie teased her and looked inside. The flowers withered and dried, lacking in care for a few days. She wasn't sure how the flow of time between dimensions worked, but she had only left for no more than two days. She recounted the possibilities as she traversed through the cave, intensely observing the quiet tunnel, hoping to find some clues.

There was no sign of fighting or struggle. The cave was untouched and not messy. Her family didn't spontaneously disappear. Something forced them to leave before she could come back, something terrible.

In the living room, Iris found a piece of paper on the dirt table. Beside it was a black feather, which strangely held the paper in place. She promptly walked into the room, picked up the message, and read it.

"Dear Iris," Lenmia wrote, "I'm sorry we couldn't wait for you. After you left, a group of humans found our cave. We won and got a new member—Tardi's human sister—but they managed to send a signal and expose our location. For your safety, we've decided to lure them away. We are now on the run, but don't worry; we're fine. I know this forest like the back of my hand and will be waiting for you at the Labyrinth of Love, in the main layer. There is a map you can use in Vilia's hideout. Please, be careful."

The Labyrinth of Love was where the strong Monster Girls resided. To enter it was to venture deeper into the Sinking Dark Forest, treading the ominous scenery, fending against the savage Wild Beasts and the wandering Monster Girls. Iris wasn't sure if she could do it alone, and suddenly she was overcome by her longing.

"No, don't panic," she said and slapped her cheeks. Her slime hand merged and popped out on the other side, sending bits of her mass flying. "Examine this first; decide what to do later."

Iris stared at her chest. Inside her translucent mass, the Shadow Heart Core floated motionless, permeating her slime with a constant flow of Corruption Power and foreign energy. It pulsated like a fleshy heart.

Iris sent her consciousness into it as if it were her true heart.

Despite recalling Vilia's memory, Iris found nothing about Lilith or the Shadow Heart or the Shadow Plane. It was a secret too ancient for ordinary Monster Girls. Maybe Sister Lenmia might know something, but Iris had to find her first. It was a good thing the Shadow Heart Core constantly nourished her body, too good for a parting gift.

"Sis Vilia, you think so too?" Iris said as she closed her eyes. "Bad or good, we'll see it soon enough."

Iris instilled her consciousness into the Core, which quivered as it connected to her soul, submitting to her will, screaming in joy for its new owner. From its darkened surface, strange power leaked out and imploded into a singularity, which infinitely expanded and consumed Iris.

She struggled, but a pleasant feeling enveloped her and filled her with familiarity, the same familiarity she shared with Gulia. She suddenly fell through the ground, descending into the dark void that existed beneath the material plane. The fall was a long and fast one, but Iris couldn't feel a single discomfort. Her body merged with the darkness, the shadow.

She emerged on the other side of the world, where everything was shaded grey.

The monochrome ruled the Shadow Plane, which resembled the great material plane.

Iris looked at herself. Though she was still a Slime Girl, her body had faded in colour except for a hint of azure and purple near the Shadow Heart Core. She reached inside herself and took the Core out, inspecting it.

As she caressed it, her soul felt fuzzy as if she were caressing her own heart. The crystal pulsated and infused into the surrounding foreign power and colours. The surrounding slowly regained a hint of saturation.

The world was familiar, but the atmosphere had turned stuffy. The Corruption Power pervaded the air like a thick fog, trying to corrupt Iris, who was too weak to absorb them.

If she couldn't control her power, she would lose her mind and become a slave to her lust. Akin to holding her breath, she closed her membrane and confined herself inside. Though it felt congested, she couldn't risk losing herself.

"There is no need to panic, Iris," a voice echoed from behind. "My Shadow Heart is the controller of this world. The Corruption Power can never taint you."

Iris spun around and raised her hands, commanding her tendrils to spread. The lady standing in front of her merely smiled and raised both of her hands. Her body was flickering in and out of existence like a ghost, a phantom of the past. A beauty in a red oriental garment.

"Who are you?" Iris said and narrowed her eyes. She blinked a few times and forced herself to believe that the red clothes she saw were real and not her hallucination. "What is this place?"

"The Shadow Plane is my creation," Lilith said. "Gulia should have already told you that. You have a piece of my heart, so I know you're my successor, fellow Foreign Existence."

"You are Lilith? But you've already fallen."

"I'm her projection. My true self fell long ago; this world is my legacy." Lilith surveyed the surrounding, evidently proud of her masterpiece. "At our height, it is easy to create a few projections."

"What do you want?"

"I know Gulia has already told you about our resentment. I also want to pass down my power to my successor in hope that one day you will continue my legacy." Lilith touched her chin and observed Iris, looking through her body and into her soul. "Very intriguing. You already have a hint of faith inside you. As expected of us Outsider, always at odds with the divine."

"What do you mean?"

"You'll understand it when you become stronger; I'm here to tell you how to become stronger." Lilith pointed at Iris's breasts and giggled. "The Core will prepare you for your breakthrough into the Transformation Phase. After that, you can come here and explore this place if you want."

"I refuse to believe that is the whole truth."

"It isn’t, but isn't it exciting if you have to figure it out yourself, more rewarding, more impactful?"

"If I die, or if my family gets hurt, I won't hesitate to abandon your legacy."

"I'm just joking, dear Iris. How can I play with my legacy like that?" Lilith wiped her non-existent tears and faked crying, smiling. "Listen carefully: there are more Heart Fragments out in the wild, some our cults hold, others the Pure Races gathered, or the demons got them. Each fragment will attract one another, trying to form the whole."

Iris's expression darkened. She arched her hand, but Lilith snapped her fingers and froze her movement. She couldn't command her palm to let the Heart Core go, though she never intended to throw it in the first place. She was just checking the reaction.

"Calm down, Iris. You have the Heart Core I prepared for my successor. How could I not give you any advantage?" Lilith casually strolled to Iris and lowered her hands, clasping the Heart Core, still grinning. "Your presence will remain invisible as long as you don't enter the Shadow Plane, and the Heart Fragments won't be able to know your location unless they stare directly into you."

"How strong should I become before I attempt exploring this place?"

"The earliest will be the peak of the Transformation Phase. This place is dangerous, dear Iris. This is where the truly strong walk, trying to unearth what we left behind."

"Can't you just show me your hidden treasure and help me become stronger? I need the power to protect my family, and you need me."

"I can only conceal you in here for so long. Time is running out. Remember, do not enter unless it is the last resort. You can bring your family here, but the Core will not protect them." Lilith's silhouette rapidly flickered. "The others are watching. They already know the Core has awakened. Good luck on your journey, my Kin."

Iris felt her slime boiling and ruptured into a burst of light. It expanded and tainted the Shadow Plane with colours until everything went white.

The moment Iris opened her eyes, she had already returned to the material world, standing in the same place as when she departed.

The world dimmed as the sun traversed across the sky and landed on the horizon. The green mountains from afar obstructed the sunlight, which painted the world orange and crimson.

The night gradually crept into the world, and Iris knew it was going to be a long one.

She looked down at her body. The Heart Core had already changed its colour and shape, resembling the slime core of a typical Slime Girl. Though its colour was dark purple, it strangely merged inside Iris's body as if it were there all along.

"Vilia, you heard her, right?" she whispered to herself, to the memory she acquired. "It was reckless dealing with the devil, but I need that power. You understand me, right? I can't afford to lose any of you again."

After a moment of justifying herself, Iris began to look around the cave, trying to find anything her family might have left for her. She had to prepare for the departure. She felt this would be the last time she saw this cave. The intuition was unfound, but she couldn't bring herself to doubt it.

It would be a long walk, spiralling down and down.