By activating the 【Master Code】, I became the master of this underground city.
According to the information I got from [Ruins Seizure], the mastermind is holed up in a building in the back by himself alone.

　As I was thinking of ......, I suddenly heard a voice in my head.

'Alert from the underground city system. The intruder's response has disappeared. --We presume you are in a hiding state due to a special item. Please be on the lookout for a surprise attack.''

　A special item would be the legacy of an ancient civilization.
　This one is quite tricky.
　It would be too much trouble for me to find the mastermind by myself, so let's use manpower tactics here.
　I open the item box and return the equipment to the Diabolos Armor.
　I focused my attention and activated my magic.

''Everyone, lend me your strength. --Necrodark.

　One by one, the undead crawled out of the ground.
　Dullahan Lord, Elder Rich, Wyvern Zombie...... all of these are monsters that are designated as Danger Level S in the Adventurer's Guild.
　Considering that there is only one enemy, overstrength is a good thing.
　However, you never know what will happen in this world.
　It's best to avoid missing the mastermind by accident, and to be extra careful.

We are now going to find the mastermind. We don't know where the enemy is hiding. I don't know where they are hiding, so I want you to stay alert. Understood?
「「「「「 goooooooooooooh! 」」」」」

　The undead let out a heroic yell.
　Perhaps, "Leave it to me! I guess that's what he's saying.
　It's quite a dependable atmosphere.

　I decided to leave the search itself to the undead, and set about fortifying the entrance to the underground city.
　Since there is only one entrance and exit, the enemy will never be able to escape from the underground city as long as I hold this entrance.

　After preparing myself, I called Charu, who was waiting in the tunnels, over to him.
　Char is a survivor of an ancient civilization and may know about the enemy's 'special items'.

''If it's a magic tool that hides your whereabouts, I have an idea of one thing.
Can I ask you a question?
'Of course. I can finally be of help to you, Ko-sama.

　Charu nodded, and a little, happy expression appeared on his face.

''The 'Demon Fog Armlet' - an extremely rare magic tool, which allows you to completely hide your appearance when you use it. However, its duration isn't that long, and once you use it once, you should have to let it rest for a while.
Does that mean he'll be revealed soon?

　I turn my attention to the underground city.
　Its streets are sophisticated, with a residential area in the foreground and a number of buildings that look like research facilities lined up at the back. The overall impression is that of a "research city" with a futuristic atmosphere.

　...... Hmm?
　I heard a distant, contested sound.
　Almost at the same time, I could hear a voice into my head.

''We've located the intruder. It will appear on [Auto Mapping].''

　I immediately activated 【Auto Mapping】.
　A map of the underground city spread out in my brain in a flash.
　The enemy's position was indicated by a blue light spot, but it was heading towards the entrance of this ...... underground city at a furious speed. It seems that they are cutting down every single undead they encounter, and their souls are returning to me one by one.
　Apparently, the other party seemed to be quite skilled.

　At this time, I had taken about 200 undead with me and had formed a formation to block the entrance to the underground city.
　The formation was semi-circular, with the Dullahan Lord on the outside, the Elder Rich on the inside, and me and Charu standing in front of the doorway.

''Everyone, be alert!　The enemy is coming!

　When I shouted, the undead prepared themselves in unison.
　Soon, a man in black appeared in the distance.
　He was running while carrying a thin longsword in his right hand.
　The distance between them was about five hundred meters.
　The Elder Rich and the others began to chant like grunts in order to perform a magical attack.

　It was that time.
　I'm not going to be the only one who has been in a position to do this.

　How fast it is.
　Obviously, it's beyond the abilities of humans.
　The man in black may be using some other special item besides the 'Demon Fog Bracelet'.

''It's speed for ...... speed.''

　I changed my equipment to a Fenrirsuit and activated the 《God Speed Blessing S》.
　In the accelerated world, everything except me appears to move slowly.
　No.
　There was one thing that was moving at the same speed as me.
　It's the man in black.
　What kind of principle is this thing moving so fast?
　When you activate [Appraisal] while wondering, the answer flows into your brain.

'Gale Long Sword (Prototype Type):'
　A long sword that transplanted Fenrir's unique skill 【God Speed】 using ancient magic technology.
　However, the transplantation technology is imperfect, and the effect time is one minute, requiring an hour's cool time each time it is used.''

　So the longsword that the man was holding was a magic trick.
　Once I realized that, it was a quick story.
　I went to intercept the man while activating the "Divine Speed Blessing S".
　The black-clothed man was raising his longsword wide to cut down on the Elder Rich, but he didn't seem to think that there was an opponent who could move at the same speed as him, and his movements were full of gaps.
　''Chance.

''--Haah!

　I took out the hikino wooden sword from the item box and unleashed a stab at the man's abdomen to gouge him out.
　However, the man in black twisted his body to evade just in time and jumped backwards in a big way.

　The man's skin was sallow and black, and all of his hair had been shaved off.
　It's a figure that has an air of an 'assassin' in the air.
　As for his age, he's probably in his late twenties.
　The emblem resembling the number "8" is carved on the right side of his forehead, giving him a somewhat eerie impression.

　I readjust my wooden sword and slash at him twice or three times in a row.
　Although it couldn't inflict a fatal wound on the man, it was successful in cutting the opponent's momentum and, conversely, pushing him to the outside of the formation.

'You can move at the same speed as me, ......?　d*mn, I was going to force my way through.
Sorry, I'm not going to let you go. -- surrender if you don't want to be hurt.
No, I'm not going to do that, you f*cking idiot.

　The man spits out the words.

"You look like you're getting carried away with being called a "dragon slayer" or something like that, but ...... I'm going to show you what the real deal is when it comes to killing each other.

　The man in black grinned and put his left hand behind his back.
　What was he going to do?

　-- The man's 'Long Sword of Gale' was just about to run out of time.

　The man's acceleration ended and his movements stopped.
　As for me, the 《God Speed Blessing S》 is still in effect, and I can stay in the accelerated world forever if I want to.

''Uh, ...... are you going to tell me what the real killings are?''

　But to be honest, I'm not interested in such things.
　We've had to deal with black dragons, rampaging dragons, and other monsters beyond human understanding many times.
　I'm full of life.
　If possible, I would refrain from doing so at all costs.

　Anyway, the winner is decided.
　I took out a lightning basket from the item box and put it in my left hand, touching the man in black.
At the same time as I deactivated the 《God Speed Blessing S》, I activated the 《Lightning Strike Paralysis S+》.

''This is the end.''
Goooooooooooooooooooooooooooooooooooooooooooooooooooooooooo!

　A pale spark popped.
　The man's entire body trembled and eventually fell to the ground with white eyes.

　All that's left is to take this guy back to the royal capital and make him vomit all over the place.