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On a patrol with Eraserhead, quirkless vigilante Izuku (known as Whisper for how silent he is) had just taken down a villain group when one ran. Eraserhead lets him go after them and the chase ends when he knocks them down and finds a crying lost kid nearby.
The kid seems to be a tourist that doesn’t know much Japanese and Izuku doesn’t know their language well either. He is comforting the kid and calling the villain and kid in when something hits him to the ground from behind. It's the kid's parent that is a vampire teaching the kid how to hunt.
Vampires are careful to not hunt in their own countries and go to others to do so when they need a lot of blood for new changed vampires or for their naturally born young that need more to grow quickly. They usually target villains or vigilantes because them going off the grid isn’t as noticeable as a hero.
The kid already drains the villain Izuku caught as he tries to fight the parent. Izuku struggles the best he can but a bite from the parent paralyzes him with venom. The kid and parent almost kill Izuku from drinking his blood before Eraserhead gets there and fends them off with a UV flash grenade and gets Izuku to an Underground agency to heal him.
Because the vampires didn't kill him, their venom flows in his blood and turns him into a vampire over several days of having a fever and being incredibly sick. Eraserhead stays with him and keeps him safe from other heroes that want to end his suffering or stop him from turning at all the whole time.
Underground heroes are more than just that, they are also known as hunters of the supernatural. Werewolves, vampires, ghosts, and ghouls, all are watched and hunted by Underground heroes if they go feral or kill outside designated areas/feeding grounds.
Eraserhead doesn't want to kill a new feral Izuku because they have been patrolling and working together for so long and see each other as partners on patrols and trust each other with their life. Eraserhead makes a deal with the other Underground heroes that Izuku will be trained as a hero at UA and kept from killing with his own life being on the line if Izuku does kill or go mad.
Notes:
Vlad King is also a vampire and Nedzu knows this. He helps Izuku with his new vampire powers and how to drink blood without people noticing, avoiding the sun with lotions and stuff, and how to handle blood on the job
Toga is also a vampire but a born one from her mother cheating, so she isn't feral until her parents block her blood drinking like in cannon.
Stain is also a vampire.
A vampire's powers depend on the person and how many generations of vampire are in their family ancestry. Someone freshly turned could be stronger if an ancestor was also one and it became like a recessive gene over generations until another vampire’s venom set it off.
Vampires existed before quirks and there is a very small chance a vampire will be born with the quirk factor to get one. Their powers are often mistaken as a quirk as with Vlad King and Toga and their blood ‘quirks’.
Eraserhead has a vampire in his ancestry which is why his quirk works on vampire powers as well as quirks. The red eye glow and floating hair/scarf are identifiers of it.
Izuku is homeless because a villain fight hit his apartment with him and Inko inside and started a fire from a gas leak and a spark. The smoke inhalation messed up his throat and a falling beam burned his neck and chest to take his voice. He learned signs as a kid because he wanted to talk to Bakugo if his quirk hurt his ears and Present Mic knew it so little Izuku had to learn too! The loudest he can get is a whisper or little whistles and clicks he makes to 'talk' to Eraserhead on patrol. His injuries never got recorded at the hospital because they kicked him out after learning he was quirkless after the rush to heal all the people hurt in the fire and fight. Izuku is marked as dead in the fire with Inko.