When Kei is done with his transformation, He ordered his manticore to get the cerberus attention away from him, when the manticore received his order it immediately leapt towards the cerberus and slammed its big body on it which caused the ground to shake due to the impact.

It became a test of strength between the two mythical beasts causing astonishing impacts whenever they slam to each other...

While these creatures were facing each other Kei started his preparations on creating a trap for the cerberus. He slit his wrist and lots of blood flowed, He controlled his blood making it form a circle and in the middle of this circle were the beasts trying to beat each other to death.

When one of the Cerberus head saw Kei making preparations it made a pouncing stance but failed because the Manticore interfered, making it extremely mad and showing a great deal of pressuring aura making the Manticore unable to move for a second..

Using this opportunity, the Cerberus jumped towards the Manticore slamming its body on it causing the Manticore to clouched down.. trying to fight back but it was all in vain because of the Cerberus unfatomable strength.. It was about to bite the manticore with its three head when..

Stabbed...

A bloody blade stabbed its body, the Cerberus look around the area to find to culprit and it saw something else..

Covering the entire fighting circle were thousands of bloody blade, covering all areas for escape..

This scene looks like the one in campione where the MC commands thousands of blade when he kisses one of his heroines.

Seeing that there are no place for escape the three headed dog started to focus its black energy into making a black barrier, it covered the entire body of Cerberus making an unbreakable defence..

Kei commanded the bloody swords to attack the enemy and the entire circled area rained with bloody swords going straight to manticores defence..

After the attack the entire area was covered with smoke because of impact, He focused his gaze on the place where the Cerberus was standing earlier..

While waiting for the smoke to be cleared out by the wind Kei and the students can hear the beating of their hearts 'tugtug tugtug'....

The smoke that covered the entire surrounding is starting to clear..

A black shadow of a big creature can be seen, it suddenly leaped and arrived in front of Kei.

It was the Cerberus, covered with blood it slapped Kei with its gigantic claws but was only able to cut Kei half on the others students perspective

Only Vanessa and Brook standing with the students while watching the fight were the only people who wasnt shocked right now..

That's correct!

Kei used his skill Kyoka Suigetsu, a skill that makes an illusion like afterimages.

Kei appeared behind the Three headed Beast and slashed his sword on its back..

But it didn't even caused a slight damage on the cerberus because of its barrier.

The blood on it's body is only because of the bloody sword, it didn't cause any damaged at all...

It opened its mouth and created a big black fire ball aiming at Kei who is still in mid-air..

When Vanessa and Brook saw this they immediately covered all the students with barrier and braced for the impact.

Brook and Vanessa never expected that this Class B beast has a destructive attack of a Class A creature, they can only pray for Kei's safety..

"Booooooooooooooooom"

Because of the black flames destructive power it caused a huge explosion. After the a few minutes

 There was the sound of sizzling, as though someone had thrown a burning torch into water.

The attack which surpassed the very definition of classes went off — it looked as though the sun had risen on the land, dying everything in his field of view was dark flames. 

The exothermic conflagration generated a rapidly-expanding wave of heat, which greedily consumed everything within its radius.

This hell of a vision lasted only five seconds, but it felt dozens of times longer than that.

Eventually, the brilliant dark world disappeared. In the wake of the vanishing pulse of superhot energy was a large circular area — vastly different from before.

Nothing outside the area of effect was affected. The trees were still trees, the academias lands was as full of vitality as the forest, and the forest itself was untouched — an extremely normal world. Good thing that the students together with Brook and Vanessa were located in the place far away from battle..

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 In contrast, the area within the circular area was charred black, converted into a dead zone of jaw-dropping proportions.

The awe-inspiring temperatures had consumed all the vegetation in the area, leaving only a few carbonized tree stumps. There were several vitrified areas on the ground which were still emitting smoke.

Standing beyond the bounds of that world which permitted no survivors, The Cerberus felt a dreadful presence washing out from within.

 It came from the only person who remained within that area. (The manticore already returned to Kei's shadow after its defeat)

 Nothing living could have survived that fatal heat.

 “Kakahaaah~ Kahaaaah~”

 That strange sound, accompanied by what seemed to be a gnashing of teeth, filtered into the Cerberus ears.

 It turned to look at its source, and saw a point of red amidst a world scorched black.

 It saw Kei with wisps of smoke trailing off his body, and a look on his face which seemed to say, “not enough gun”. His crimson pupils, filled with hostility and bloodlust, focused on the body of Cerberus.

 “Heh, you can still move even with that attack of mine. Really a slippery opponent.So do you think you can take responsibility for making me this excited?”

For the first time the Cerberus spoke;

Kei didn't respond

He slowly walked forward, his footfalls fracturing the charred ground underneath.

Step by step, he shrank the distance between the Cerberus and himself, the Harusamme in his hand cutting through the air with a whoosh which indicated that he was still able to fight.

The Cerberus did not scramble back. In a tone which called to mind an image of a champion welcoming a challenger, The Cerberus arrogantly taunted Kei:

“That was just a meaningless present. Did you like it, Kei?”

“Ahahahaha!”

Kei laughed, and his mirth came from the bottom of his heart.

“Wonderful! To think I have to kill a powerful being which two classes above me!"

Moving with a speed which put gales to shame, Kei charged, wreathed in bloodlust. The scorched ground exploded beneath his feet with every step he took. Brook’s assaults were swift, but Kei's speed was in a class of his own.

For an instant, Cerberus was grateful that he did not need to to worry on blinking because he had three heads to take turn on keeping watched, because Keiwas fast enough that it would lose track of him if it took its eyes off him for a moment.

Trailing laughter behind him, Kei continued his charge, aiming the tip of his sword at Cerberus and thrusting forward. The sword charge was originally a technique used by knights and made with the speed and weight of their mounts behind it. However, Kei's strike — made with his extraordinary strength and his awesome speed — easily surpassed that attack.  

The word “one-shot kill” could not even begin to describe that assuredly fatal blow, and it tore through the air towards Cerberus.

Suddenly Kei he stopped when his sword was about to stab Cerberus, he spoke;

"You know what, I've taken a liking to you and I want you to serve me. I'm a yakuza not a samurai, I don't want to kill strong people, I only want strong allies. So will you lend me your strength?"

Said Kei while posing like Nura Rikuo when he invited one of Haguromo-gitsune's ally.

When the Cerberus heard Kei, It started and laughing;

"HAHAHA, interesting. Very well, as long as you can defeat me. I will serve you till the end of my life!"

When Kei heard its reply, he then smirked and..

With a smile of victory, Kei evoked an Impure Shockwave Shield. A wave of force — colored reddish-black like clotted blood — spread forth, disintegrating the gravity orb upon contact.

This was one of Sigilia family's skills, which combined offense and defense.

“Cheh!”

Cerberus clicked his tongue at this.

Kei laughed at the expression Cerberus was making, and then he showed off another special skill of his.

A gigantic sword appeared in his hand. It was well over three meters long which is really large.The aura of purity it emanated proved that this was no ordinary weapon. It reflected the rays of the sun in its silver radiance, producing a beautiful and eyecatching display. It looks like one of Asakura Yoh's O.S when he uses futsu no mitama together with Harusamme

“Oh… I’ve never seen that before. Did you summon it or something?”

"allow me to introduce it to you. Its one of my skill named Oversoul!”

Kei suddenly vanished, It is also one of their race special skill; Blood Teleportation

The User can teleport/transport between any two large enough concentrations of blood, usually living people are used as both enter and exit point but a pool of blood would possibly do.

He suddenly arrived behind Cerberus where a pool of blood was located and aimed the big sword as he laughed inwardly.

“Guwaargh!”

—And it did hit, piercing Cerberus chest. But he made sure that it was stabbed in a place where its not too dangerous.

After the match, the Cerberus was looking at Kei like a loyal dog which greatly defies the image it had shown earlier..

"I've never seen someone as interesting as you before and since I said that I will become your subordinate when I lose, I'm gonna keep my word and form a blood contract with you"

Blood is binding — Any magically contract made in blood must be honored, on pain of death. If you make a Blood oath, and swear that "if you break your word, may the earth drink your blood!", and break your word anyway, you may find yourself six feet under.

So when Cerberus said that he wants to make a blood contract Kei was astonished..

Kei immediately slit his wrist and let the Cerberus lick it, he spoke;

"From today onwards, you will become part of my night parade of hundred creatures, your enemies shall become my enemies and my allies shall become your allies, this vow I promise to fulfill"

Then Cerberus also made it's vow;

"From now on you shall become my master, No matter what you order me I shall follow without any complaints and promise that I will become your most loyal subordinate until the day I die"