The Undead Summoning Formation was Hel's first layer of filtering.
This magic circle could summon undead within its radius to a location designated by the caster.
Of course, there was a restriction: only ownerless undead or undead already bound to the caster could be summoned.
This first layer existed to prevent Hel from accidentally summoning an undead belonging to the Witch of the Underworld, which could reveal her identity.
"Then, let the rest of the spiders get to work," Hel whispered to Niv.
With a wave of her small hand, Niv sent countless mechanical spiders rushing to the surface. They immediately began digging tunnels.
At first, the excavation was slow — the entrance only allowed a few spiders at a time.
But soon, more and more spiders poured into the hole, speeding up the digging process exponentially.
Dust flew everywhere, only to be quickly transported into the Undead Space by the spiders waiting outside.
Hel and Niv, meanwhile, pitched a simple tent nearby, knowing that neither the array-deploying spiders nor the digging ones could finish their tasks quickly.
It looked like they would spend the night in the forest. Hel held Niv close as they slipped inside the tent.
That night, Hel slept soundly, resting her head on Niv's lap, but Niv didn't sleep — she needed to micromanage the mechanical spiders.
Deploying the formations wasn't just about speed — they had to be precisely placed with no gaps.
Any hole in the coverage could let undead slip through, reducing Hel's catch and potentially threatening Heim's defenses.
This explained why they had to stay overnight in the forest.
Additionally, they needed to avoid detection of the mechanical spiders and manage the excavated soil — tasks that required personal attention.
By dawn, the forest had transformed. Birds chirped crisply, waking Hel from her sleep.
"Ugh… what time is it?"
Hel rubbed her sleepy eyes, her voice soft and muffled.
Niv patted Hel's messy hair affectionately.
"Master, it's 6:35 a.m. Would you like Niv to give you a morning bite?"
"Yawn… Niv, don't copy that weirdo Lily."
Hel pushed Niv away gently, pinching her soft, baby-like cheeks in the process.
"So, how's the work going?" Hel asked.
"Report, Master," Niv replied. "Ten thousand formations are deployed, and the underground base has been fully excavated. Shall we take a look?"
Niv led Hel to the dug-out tunnel entrance, a yawning black hole that seemed impossibly deep. Hel instinctively swallowed hard.
"Forget it — we're not going down to work anyway."
Hel opened Undead Space, pulling out a small skeletal mage. The little skeleton blinked, clearly confused, as Hel locked eyes with it.
[Intermediate Death Magic: Possess Undead]
In the next instant, Hel gained a second perspective, seeing through the skeletal mage's eyes.
She had used this spell many times before, so multitasking between her body and the skeleton was manageable.
Hel placed the skeletal mage at the tunnel entrance.
Through its vision, the hole appeared even deeper, and she instinctively stepped back — colliding with Niv behind her.
"Master, are you scared? Should I help you?"
"Never mind… I'll do it myself."
Hel took a deep breath, stepped forward, and kicked the skeleton into the hole.
Immediately, a strong sense of freefall transmitted from the little skeleton to her, making Hel's stomach churn.
"Master, I said I'd help — by sending a mechanical spider to carry it down," Niv said awkwardly, pointing at the ready spider squad. Hel felt a twinge of guilt.
"Why didn't you say so earlier?"
Hel scowled, disconnecting briefly from the skeleton's perspective.
"It's fine — faster this way. A fourth-tier skeleton won't die from a fall."
"But Master, the base is roughly 120 meters underground…"
Hearing this, Hel jumped to the edge of the tunnel.
She stared down at the seemingly endless darkness and imagined the skeleton plummeting to the bottom. Her heart ached.
One hundred meters vertical — that poor skeleton would be shattered into pieces.
This was a creature she had painstakingly prepared over three days, and now it was gone?
Would she have to start all over, working another three days straight?
The despair she felt was like completing a whole day's digital art project, only to have the computer crash before autosave — sheer helplessness.
"Master, don't worry. Mechanical spiders are stationed inside the tunnels to catch it. By now, it should be safely at the bottom."
"Really?"
Hel quickly reconnected with the skeleton's vision.
The world appeared dim green, representing its night-vision perspective — essential for any undead.
Looking down, she saw the skeleton being carried by two mechanical spiders. After passing the last segment of rocky tunnel, the view finally cleared.
The underground base was about 5 meters tall, covering roughly 1,200 square meters.
All walls were reinforced with thick red-copper plates, and dozens of sturdy copper pillars provided central support.
"You're telling me this was built in one night?"
Hel stared in disbelief at the base, glancing at Niv.
"Yes, Master. Mechanical spiders specialize in excavation and construction. After digging the tunnels last night, I personally went down and opened Undead Space, allowing the base to be completed so quickly."
Hel nodded reluctantly, stretching and moving her limbs before speaking with slight irritation:
"Fine. Seal the entrance. Once I finish drawing the summoning arrays, the real work begins."
"But Master, are you sure? Wouldn't it be better to combine the undead using Death Seals before filtering them?"
"Can't do that — we're feeding in someone else's territory, so we have to be careful. That's why I insisted on coming to this forest near the Watchtower Wall — it reduces suspicion."
"Understood," Niv nodded.
Hel then set to work.
For the next phase, she would control the little skeletal mage inside the sealed underground base and inspect each undead's data entry one by one.