Izuku had no clue what was happening to him, but he was smart enough to know that whatever was happening had something to do with last night so he sat down and retraced his steps. He remembered what Izumi told him at school, the sludge villain and meeting All Might and learning who All Might really was and the secret of his quirk.
He distinctly remembered his suicide plan just from how sure he is he would have done it. Now that he's calmed down a bit the thought scared him. Was he really willing to die, just because he couldn't be a hero!? There are other options, he could just run away or something!
The next part was hard to remember. He remembers he was attacked, but couldn't remember much about it. He remembers it was in an alley, why he was there he couldn't remember for the life of him. He could remember even less about the actual attack. the only thing he remembers clearly was the glowing red eyes and fangs of his attacker.
After that, his memory was blank until he woke up in the alley with intense... would thirst be the right word? No, this was something different. Normal thirst wasn't that all-consuming. He remembers barely being able to think like that. He could think about was the intense need for something. He remembers waking up to see one of the gang members he'd worked with in the past. As soon as the man saw Izuku's open eyes he looked shoced and ran for it, he didn't make it far. Izuku chased him and drained him of all his blood. He didn't want to believe it but it all felt so real, and he had the evidence in the form of his bloody fangs to prove it.
He needed to double-check. He pulled out his phone and checked the news, his hope for it all to be a dream was dashed at what he saw. The guy's body was found drained of blood in the exact same spot he remembered leaving him.
He read they had security camera footage of the event and his heart dropped. What if they saw his face!? They'd know it was him and he'd be sent to jail! He started to panic, the thought of going to jail terrified him. He checked the footage to see if they could see his face but was surprised and confused. He couldn't see himself at all. He clearly saw the man running scared of something. All of the sudden the man was attacked by what seemed to be an invible attacker that Izuku knew was him.
How? He could clearly see himself on his phone camera, so how wasn't he on the security camera? There's so much he didn't know. Was this temporary? Permanent!? Is this his quirk or someone else's?
He checked his watch and realized he was going to be late for school if he waited any longer. He rushed to put on his school uniform,cleaning his face and tossing the tissues in the garbage bin, and packed everything he needed for class. When he looked back at his watch, only a few seconds passed. Does he have super speed? Deciding to test that idea he ran to school today. What would normally be a 30-minute full-speed run was now done in barely a minute, and he wasn't even going at his full speed.
When Izuku got to school he checked the clock and saw he had a few minutes to himself. He decided to go to the library and looked up everything he could about his new quirk online. He searched for anything related to fangs, blood-drinking, his lack of a reflection, and his super speed and strength. All questions pointed him to one result, vampires.
Did he awaken a vampire quirk? How? Neither of his parents had vampire quirks or anything even remotely similar so why did he? The memory of what happened in the alley came back to him. He moved to a darker section of the library, pulled out his phone, and looked at himself. Both of his eyes glowed blood red, just like the man in the alley.
Did he somehow transfer his quirk? The idea would seem crazy to Izuku had he not learned about One For All and All For One so he knew quirk transference was possible. If that was the case then it means Izuku now has a quirk.
Izuku spent the rest of the day secretly testing everything he learned online. Sunlight didn't seem to affect anymore than a mild tingling sensation but silver and garlic did. Silver hurt to look at, like he was staring at the sun, and it burned to the touch. Garlic smelled repulsive and when he tried to force it down he ended up puking what looked like old blood in the bathroom.
As soon as he puked up all the garlic he noticed something, he was very pale and thirsty. He must eat, he must feed! The sunlight from the bathroom window made his skin burn as it bubbled and seemed to slowly turn to dust. The door to the bathroom opened and new prey walked in. He hid in the shadows, waiting, and when his prey was in a stall then he attacked.
It tastes wonderful! He wants more! More! MORE! STOP! Izuku all but ripped himself from his fellow classmate when he realized what he was doing.
The boy he fed on was ashen. He'd been knocked out as when Izuku pounced on him from behind he also knocked his head into a wall. He checked and found there was still a pulse. Thank god. Izuku also noticed that on the side of his neck where there should have been fang marks there was nothing but two scabs. Did he cause the wound to scar over?
Izuku went into a nearby classroom and told the teacher he "found" his classmate in the stall like that. He wasn't worried about getting caught, this school didn't have security cameras. When he was brought to the nurse he was found to have severe blood loss and a mild concussion. He was taken to the hospital to get a blood transfusion.
Izuku looked at his hand where the light had burned him and found no scar. It was healed. He went back to the bathroom to look for the source of the light and of course, it was the sun. "Okay, so sunlight only hurts me when I'm hungry," he muttered.
He didn't do any more experiments for the rest of the day out of fear that he'd accidently hurt one of his classmates again. When class was over he mentally went over the day and realized something. Izumi and her gang hadn't messed with him all day. That realization should have been relieving, but instead, it was worrying. In the 10 years since he was diagnosed quirkless he'd never been given a break. Maybe this was a fluke. They'd be back to their normal selves tomorrow.
When Izuku got home he immediately went into his room. He lay on his bed and thought about the day he had. Why was he so calm about all this? He'd killed someone, he knew that, and yet he barely cared. Maybe this was something to do with his new quirk, maybe it affected him psychologically. Does that make him a bad person? He thought back to the kid in the stall, he stopped himself from killing him. That must mean he didn't want to kill him.
He wasn't a bad person, he just needed to get better at controlling his quirk's new urges.
"So what do you think dad!?" Izuku heard Izumi's voice... from all the way in the living room. That shouldn't be possible, his hearing was messed up from all of Katsuki and Katsumi's explosions.
"Hm, I don't know Izumi. While what Izuku did was kind, it was also something almost anyone would do in that circumstance," Izuku heard his father respond. He could not just hear his father in the distance but also smell him. His blood was just right there. Could he get it before he got the chance to go full All Might? Izuku didn't notice his eyes turning a glowing blood red color as he thought about that.
"Come on dad! We've known Izuku since he was born, you know how much he wants to be a hero!" Izumi replied.
"I know Izumi, but this is not a decision to be made lightly," Toshinori responded. Izuku was now at full attention. Were they talking about giving him One For All?
"Hmph, fine! I'm sure before school's out Izuku will prove he's hero material!" Izumi replied. Izuku couldn't help the tears rolling down his face. Izumi cared about him? Why like this, why now? The same girl who'd told him to take a swan dive off the roof was now telling All Might to give him his quirk!? This made no sense.
Izuku thought about it and realized that if he didn't deserve One For All before, he certainly didn't now that he was a murderer. If he doesn't get control of his quirk soon, he might do it again. He remembered when Toshinori explained One For All and knows that it also enhances the quirk of its user. He didn't want to consider what would happen if his quirk's "thirst," for lack of a better term, was enhanced.
With that thought done Izuku grabbed a pair of gloves, made his way to his window, and opened it, before leaping outside. Once he was outside on the grass he took off in a burst of unnatural speed. He noted the night was so much clearer than it usually was. Another thing to add to the list. It didn't take long for him to get deeper into the city but once he arrived he felt his thirst coming back. Luckily it wasn't as strong as the garlic incident so he still had his rational thought but he was still so thirsty.
Izuku did a quick test before he went to deal with his thirst. He took out his phone and pointed the camera toward himself to see he could still see himself in the video. He then tried to will himself to become invisible to the camera. Despite not knowing what he was doing it worked. Were quirks this easy to use for everyone? Izuku disregarded that thought in favor of finding something to sate his evergrowing thirst.
After checking to ensure he was invisible to the cameras he quickly made his way to one of the gangs he knew of in the city. This gang was well known for killing indiscriminately. To sum it up, they wouldn't be missed if he accidentally killed one or two of them. If Izuku was still in his right mind he'd be disgusted at himself for even considering killing any of them.
It didn't take long for Izuku to finish up the gang. With the darkness and Izuku's insane speed, they didn't even have a chance to scream. Thankfully Izuku still had enough self control not to go to kill them. He knocked them out quickly and quietly by pushing them into walls at an insane speed. It was so fast that none of them even reacted. Izuku suspected he was done in seconds but he couldn't confirm that as he had no way to accurately measure himself. One thing Izuku saw, was a black misty trail trailing him as he ran. A normal person wouldn't be able to even see this as the mist would blend in with the darkness, but Izuku's vision was no longer normal.
With all of them dealt with Izuku made his way around and drank his fill, making sure to keep them all alive. By the time he was done he felt amazing. He hoped that this much blood would keep him sane for a few days at least. Izuku then took a phone from one of the gang members and used the face ID to access it. With the phone open he called the police and told them his location. After that he leaped, jumping from wall to wall until he was on the roof.
From there he watched everything, hidden by the darkness as the police arrived and the men were taken to a hospital for blood transfusions and a check-up before being taken to the police station. Later on, Izuku would watch the news and learn that the men really did only see a dark blur running between them before being knocked out.
The blood he'd gotten from those men lasted him a week. In that week he'd taken notes on everything he learned about his new quirk. He has enhanced speed, strength, and senses. He seems to be able to naturally be able to emit a black mist from his body that seemed to blend him into the shadows. He also heals at faster speeds. Silver will burn him on contact, and the sun will burn him if he's thirsty. When he's thirstier his skin turns paler than usual. Burns he gets from silver or the sun will not heal normally. Instead, he needs to drink blood to heal those wounds.
He learned about that last bit when his week of blood was over. He left his house in secret again, but accidentally got something akin to a sunburn. Apparently the sun starts hurting long before he looses his sanity. He eventually found a man robbing a store and drank his blood and that's how he learned about blood healing sun-based burns.
The only other thing he learned was that he didn't need to sleep anymore. He still could if he wanted to, but he didn't need to. He didn't get hungry or thirst as others did anymore. In fact he couldn't even taste normal food anymore. The only sensation he got was the texture. It seems he only needs blood to survive now.
After his first "hunt" dealing with the gang, he became more of an opportunistic hunter. He didn't plan his a as attacks much anymore, just went to a random area he could get to and found someone committing a crime. After he got his fill he'd spend the rest of his time looking for other vampires. If this vampirism quirk could be spread, then there must be others with the quirk too. He has so many questions he needs to be answered. It didn't take long before he found a young-looking blond vampire with messy twin buns.