It's been a week or so since your first encounter with the Devils and the Fallen Angel that you destroyed without hesitation

You had been meditating in a suppressed form, mentally training in your mind as the night sky was clear as crystal

Meanwhile

Rias and her Peerage were currently fighting up against someone who was out of their league, a mutant Fallen who had electricity powers and multiple weapons

Kiba: *Pant*...*Pant*...just who is this guy...?!

Koneko: His power is insane...!

Issei: This guy is nuts!! What do we do Rias?!

Rias: I don't know...We tried retreating...we tried fighting back...we tried to settle things peacefully...but nothing worked...!

Everyone looked in fear as this incredible adversary emerged from the smoke, he was wearing a black jacket with a sword and throwing axe in hand, his powers were activated as well

Issei got his boosted gear ready for battle as Kiba and Koneko followed his act, readying themselves for round two

Akeno suddenly spread her wings and flew into the air, getting her magic ready for destructive use

???: You really think you stand a chance against me...? You stupid Devils are all the same...

Issei, being the most inexperienced of the bunch, was already tempted to attack by his words and ran towards the figure as Kiba ran along side him

Koneko soon joined them as Akeno raised her arm, summoning a magic circle about to unleash some electric chaos

Kiba charged in and swung his sword as the man dodged and completely turned his body into blue lightning, bolting last Kiba and electrocuting him on contact

Kiba screamed and fell to the ground as Koneko was his next victim, charging towards her as a bolt of blue lightning and reverting back, landing a kick to her stomach

Koneko's eyes slightly widened and only stumbled a bit, being more durable than the rest due to being a rook, the man suddenly emitted a powerful shockwave through the foot that was digging into Koneko's gut

The shockwave was more than enough to send her flying back

Koneko skidded across the battlefield and crashed into a building as Issei charged towards the man with a red gauntlet in hand

BOOST

Issei threw a punch as his fist hit the man's face, he smirked for a short moment as he moved his fist, only to see that it had absolutely no effect on him

Issei: W-Wha...?!

???: This is what the great "Red Dragon Emperor" has been reduced too? I came here looking for a challenge...not a Red Dragon Punching Bag...

The man suddenly grabbed Issei by the neck and shocked him with 50% of his power, this was more than enough to place him out of battle conditions

He tossed Issei away, now not being able to fight, the man was about to walk away with disappointment when he was suddenly struck by a large bolt of yellow lightning

Akeno: You can't just hurt my friends and just walk away~

???: Just did...

Akeno turned around in shock to see the man behind her, not harmed in anyway. The man raised his arms and sledgehammered her to the ground as he landed on his feet

Rias: This is really bad...! He's too strong for any of us...!!

Back to You

You were still meditating in a private spot as you suddenly felt a large energy nearby

Being a Saiyan Deity, keyword Saiyan, you were of course interested in seeing who was producing this much power

You suddenly warped to the location of the person with this large power, finding the same group of Devils that you first encountered

Y/N: What do we have here, I'm assuming that you're a fallen?

Rias: It's you...!

Y/N: Yes, I'm Y/N...

Rias went wide eyed as you faced the man with a straight face

Rias: It really is him...!

???: So you think you can beat me?

Suddenly the man began to emit electricity from his body as he turned and faced you

???: Then come and fight so I can prove you wrong...

You suddenly began to power up, gaining a F/C aura as the area began to shake, everyone went wide eyed as you have the man a glare

Rias: What's with this power...?!

Y/N: HAAAH!!!

You suddenly transformed in a blinding flash of F/C light, everyone opened their eyes to see you, but you were different, you had transformed into another elemental form, one that had Fire and Water combined with the Super Saiyan 3 transformation

You wasted no time in attacking as you appeared behind the man with your fist coated in a scorching hot flame, you shot your fist forward and struck the man, making him go wide eyed and coughing up blood

Man: GAH...!!

You suddenly screamed at the top of your lungs, throwing your arm forward with your fist still in the man's gut, sending him flying back

You suddenly transformed into your assault form and cupped both of your hands forward

Y/N: BIG BANG...

The man crashed into a wall as you widened your eyes, causing water to erupt from the ground, engulfing him and solidifying into Ice, trapping him

Y/N: ...KAMEHAME...

You suddenly disappeared and reappeared on front of him, ready to blast him with one of your most powerful attacks

Y/N: ...HA!!!

You screamed with an empowering voice as you unleashed a powerful beam of blue energy from the palms of your hands, engulfing the man in a bright blue light

The man screamed in pain as you increased the power of the attack, completely overwhelming him with an explosion as everyone else watched in complete fear

Kiba: After all we've done to damage him even a little bit, he handles him like a walk on the park...!

Koneko: He's crazy strong to do something like that...!

The light soon faded as you walked up to the smoke that would eventually blow away, revealing the man, on the ground, covered in wounds

Y/N: I'm giving you two choices, you can either give up...or you can die...it's really your choice...

???: Awesome...!

Y/N: Hm?

???: Such...Awesome...power...!! Please...let me...call you...boss...!!

Y/N: Oh? You want to join me...? Then tell me...what's your name?

???: ...It's...Kai...

Y/N: Kai...A simple yet powerful name...I like it...well then Kai...you can join me...not as my servant...but as me Teammate!