Just to clarify, the title said MASSIVE CROSSOVER, you can cleary see there isn't any harem in the title, but there is a harem, it's just going to be small (like maybe around 5, you can make suggestions on who the next 3 you guys would want.) And I'm sure as hell not going to make a massive harem and 10 pages that consist of girls that's in the harem as that may ruin the story. Also, the reader isn't ridiculously overpowered, he's OP, but can get injured and hurt. Enjoy.

I'm not really that good with humanoid monster on non-humanoid monsters, so sorry if it's bad and rushed.

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3rd POV
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The symbiotic beast loudly roars before jumping foward to strike the trihexa and did something that took the entire academy to do.

Push trihexa back.

The force behind it pushed everyone back, including a gew close remaining building. Trihexa growls as it regains it's footings and gives consecutive slashes at RN, who was able to dodge all of them before stabbing his clawed hand into the beast's chest, didn't go too deep, but deep enough to draw much blood.

The beast made a small shriek of pain before kicking RN over a large distance through buildings, but he got back into the game and retaliated with a slash across the beast's mouth, but it dodged at the last moment and attempted to bite RN, only to bite an after image from him dodging in the nick of time.

While on the ground, RN slashed at trihexa's feet to knock it off balance, which worked, but quickly regenerated and kicked him once more, but even further away. RN spat out large quart of blood before getting out of the crater he was in, only to get slammed by the quick trihexa, then got caught by its jaws and got jerked back and forth.

RN growled in pain as he felt the beast tearing anti off from his skin, so he protrudes spikes to stab trihexa's face, freeing him from it's jaws, then he got to a considerable distance and started shooting spikes from his arm at the beast as if they're bullets. The deadly spikes were tearing at trihexa's hide almost as if it was paper as RN began to realize something.

RN: (this thing's holding back; or atleast not taking this seriously.)

(I'm changing imoibrí into anti to be easy)

Anti: (well what're you gonna do about that, champ?)

RN: (simple, I'm going to kill it as fast as possible so that it won't somehow power-up and maybe kick my ass.)

He stopped firing at the beast to let it recover and ran on all fours over to it. It was slowly getting back up almost fully healed, but was soon kicked across the jaw by RN, who immediately grabbed the beast's horn to twist it's head to make pass further than a full 180° angle, which made a loud, sickening crack that made many spectators wince.

The beast roared and used it's aura to a shockwave that blow back everything and everyone in range, especially RN, to a long distance so it can get some breathing room from the heating battle with the new combatant.

the spectators were in awe and shock at the display of power between monster and the new face they've could've sworn that they met him before.

Just what was he?

Katsuki was at a safe distance, being amazed at the display of the weakling of union turned powerful being single-handedly taking on the 666 beast. If there was someone she wouldn't mind being number 1 besides her, it would most likely be him.

Back to the fight, the spectators were recovering from the shockwave while The beast was fully healed while looking at the spot it blown it's opponent and shriek at him, but what confused most about the shriek was that it sounded like it was a competitive one, like it wanted to fight him more in particular. They were under the impression that trihexa was a mindless beast.

Soon enough, RN limped out crater with a sight that made others concerned and made them want to get him off the battlefield; his arm was bent in the wrong ways and his leg wasn't any better as he continued limping towards the beast despite the protest of others, who thinks he was insane and ignorant for still going at the beast despite his current condition.

All might: Stop! You're too far injured to fight against--!?

All might: oh nevermind. carry on, strange fellow.

As he finished, RN jumped high to the air and dived at the beast to deliver a barrage of kicks, slashes, and punches. The beast felt pain but ignored it and rushed to RN head-butted him into the ground and fired a stream of unstable energy directly at him, sending RN further into ground, which made small earthquakes and pillars of energy erupting from random spots of the ground.

The devastating earthquake launched everyone high in the air right afterwards. The dust cleared to show the beast standing in the crater before it backs up to see what damage it done to RN, who was on his knees Heavily breathing with his symbiotic hide scorched, revealing his arm, half of his face, and torso area for the world to see. Those who seened it have have gotten into shock, but mostly izumu.

Izumu: RN!?

Rias: what!? I killed you!

RN raised his head to the beast staring down at him and growled before anti covered him back up and uppercutted it, continuing the fight. Neither combatants were winning or losing, but RN and a few spectators realized that the beast has been getting more and more cognitive the longer it fights RN. They've seen it try strategy and such while fighting against its current opponent Both he and the beast temporarily halted the fight as the former spoke.

RN: Beast, i already know you're no longer the same mindless monster that was fighting union moments ago, so you must atleast be able to understand what I'm saying.

The beast tilted its head and then chuckling a little before that chuckling becomes full-blown demonic laughter that send a chill down everyone, except RN's, spine. What the beast did next, shocked the spectators.

Trihexa: it's all thanks to you. not sure how, but because of that, I'll reward you a quick death.

Almost everyone was taken back by the distorted, but understandable voice of the beast.

RN: no thank you.

Trihexa: RN, was it?

RN: yes.

Trihexa gave him a wicked grin.

Trihexa: good to know~

They continued to battle as those of the supernatural faction were in further shock. They never knew that the beast was even capable of speech. The battle rages on as many had to flee to longer, safer distances. Meanwhile, the man that followed RN was at a safe distance on the top of a building, starting in awe at the ever-raging battle.

???: ok, so whatever that guy has, is more powerful than whatever semblance, sacred gear, quirk, and etc could ever dream of. What the hell does that guy have? Feels like I've seen something like it before.

As soon as he finishes, his phone ranged. He took his phone out and read the caller ID that said "mother".

Mother: Honey! are you alright!?

???: whoa, whoa, whoa! Calm down, I'm perfectly fine.

Mother: oh thank heavens! There-there were all these flashes in the skies, earthquakes, and-and--

???: i said I'm fine! Look, I'm coming home in a little bit and help you around the house, ok?

Mother: if you promise to get home safe, seiketsu.

Seiketsu: ok, i promise.

He hangs up and watches intensely back at the fight to see both trihexa and RN heavily breathing and badly damaged almost to the point of whoever gets the next hit wins. He really hoped that RN would win, so that not only he could observe him more, but also to keep the genocidal monster from killing all life on earth.

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RN POV
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Damn, this thing's tough! I keep hitting it hard, yet it keeps coming back up, but i guess the same could be said about me though. I need something to put it down for the count, but what?

Trihexa: wow, I'm never had so much fun before!

I gotten confused by how somewhat feminine the beast's voice gotten considering that it was all a bit distorted moments ago.

RN: that's quite the feminine voice you got, so should i call you by female pronouns now?

Trihexa: you can go ahead.

Anti: (so it seems that she's been absorbing some my healing abilities, which been clearing her chaos-clouded mind and made her even more capable of intelligence.)

RN: (great, now it's a smart monster. But i thought it could only heal physical sicknesses and wounds?)

Anti: (well it seems i could also heal mental health aswell; now that's a new thing i learned about myself!)

RN: Hey, trihexa was it?

Trihexa: yes it is.

RN: let's say you succeed and kill all life on earth, then what? There wouldn't be anything to do when you're by yourself.

Trihexa: i. . . Haven't actually thought much about that. Maybe I'll keep you alive as my chew-toy and straight up kill those guys behind you.

She points to the recovering spectators, who paled greatly.

Asia: please don't let it kill us.

RN: just so you know, i don't care about any of you guys, even if you died, i wouldn't really care.

Most of the spectators flinched at the blunt and cold statement i gave them, while trihexa just laughed wickedly.

Trihexa: damn! I thought you were some sort of hero that's with theses weaklings.

RN: I'm no hero, I'm just a guy who can't seem to catch a break. First, Prius Gregory and her harem over there--

Rias: *from a distance* IT'S RIAS GREMO--

RN: PRIUS. GREGORY! and her harem over there picked a fight with me for no reason at all and almost ruined my night, the next day, my friend had to bribe me ice cream just so i can get out of bed and help her with something, and now i have used the whole hour i used fighting you, could've been used for taking a nap. Now don't get me wrong, this fight has been great and I've never had to try this hard when I've ever fought, but still.

Everyone sweatdropped at the events that are happening. One moment, it was a battle that decides the fate of all life on the planet, now it's just a man complaining about having his time wasted and the supposed end of the world was being somewhat chummy.

Trihexa: sounds rough.

RN: you have no idea, Tia.

Trihexa: Tia?

RN: well saying your full name is already tiring, so i thought it would be more convenient to give you a nickname.

Tia: very well then, you may call me that.

Anti: (sooooo did we forget we're supposed to fight?)

RN: (anti, please tell me you have a way where i can finish this fight faster; She might overpower us if we keep fighting.)

Anti: (maybe you should try asking her to go away nicely.)

RN: (. . . Ah fuck it.) Hey tia.

Tia: what?

RN: could you please go away and we do this another time?

After those words came out of my mouth, most of the spectators looked at me as if I'm a jackass, katsuki is cussing me out with every single cuss word there is, even the unknown ones, and despite tia only having a maw with sharp teeth, she looked dumbfounded. This went on for a moment before she busted out laughing, making me think she's gonna say--

Tia: alright.

Wow, that somehow worked!

Tia: but don't think I won't be coming for you first!

Aaaaand there goes my sleep schedule.

Tia opened an evil-looking portal, and turned to me before going into it.

Tia: just know, no matter if you have the entire world of whatever, you will not be safe from me. . .

Anti pealed away and merged back into me as i gave her a blank stare with my exposed face.

RN: i don't care. But if you want to go directly to me, come here.

I waved the beast to come over and she did. Once she was close, i whispered where to meet to her ears and made sure no one else heard it, much to the others annoyance.

Tia: hehehe, I'm counting on it.

Then we parted ways as she went to the portal to god-knows-where and leaving me with union, the people i didn't even ask to see again but received it anyway. There was an awkward silence, so i decided to walk off and gave a half-assed wave.

RN: . . . Bye.

Izumu: Wait!!!

I heard the sound of running from behind, so i looked over my shoulder and gave izumu an annoyed glare, which made her stop in her tracks.

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3rd POV
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Seiketsu had just finish watching the whole fight between two powerhouses and threw away an empty bucket of popcorn and exited the building via fire escape and walked away with one thought.

Seiketsu: (that man holds the key to true evolution. And I'm going to have it.)

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So i finally updated this book, now it's time for the symbiotic Threat.

Seiketsu aiden is an oc ThatMemeMaker put in, and i thought that this would've added to the plot and such, So thx.

So how did i do?