(Crack idea) Izuku and his tall vampire mommy and three bug gremlins.
You see, when Alcina Dimitrescu and her three daughters, Bela, Cassandra, and Daniela, died in the other world, the last thing they expected was to be reborn in the world of My Hero Academia, which in their world was a piece of fiction.
What they really didn't expect was to be reborn in one of Daniela's AU fanfics where they were reborn as Izuku Midoriya's quirk, but here they are.
Izuku himself had never expected to awaken a quirk, not only because he truly believed himself to be quirkless, but also because he wasn't human at all.
He is an eldritch abomination of umphantomable power, but the quirk he does not have is called the King in Green, because it forces Izuku to wear a long hooded robe and mask, because it is the only way he is visible.
Without the robe and the mask, his form is inconceivable, and sentient minds simply refuse to acknowledge it; it's not like he is invisible like Hagakure. Is it that the collective subconsciousness of every intelligent being in the world does not compute him, just like your brain can see your nose but edits it out to not hinder your vision?
If he is wearing just the mask, all you can see of him is a floating mask, and if you look at his face when he is wearing the hood but not the mask, you are hit full force by eldritch insanity and go completely mad.
When he is wearing his robes and mask, he can be described as a bag of tentacles; he is a tentacle monster, not like Shoji, who has a heteromorphic quirk that gives him tentacles. Izuku is a roughly human-shaped mass of wriggling tentacles with no end or beginning. He is also the gateway to a non-Euclidean pocket dimension of madness he rules, and he can and will swallow people into it by using his tentacles to drag them under his robes, just like Raven did to Dr. Light in the Teen Titans. He can also devour minds mid-flayer style, but that's neither here nor there.
He has all the powers you would associate with an eldritch being, but the ones he sticks to are umbrakinesis, which allows him to create and command shadows, which he uses as portals. He can move through shadows; he reaches things through shadows, and he can use the shadows as a door to his limbo of madness dimension, which is called Carcosa, the realm of eternity and madness where he rules as king.
As you can guess, he underestimated what kind of universe he was on, because in this universe, he did have a quirk, but his quirk was a cunt that required a very specific activation requirement.
His quirk, the real quirk, is called Dimitrescu's Legacy and activated when he inherits his grandma's vineyard.
When his quirk activated, it materialized everything: Dimitrescu, the castle, the vineyard, and above all, the Dimitrescus themselves.
Upon being reborn as Quirk users, Alcina and her daughter noticed a few things that match Bela's fanfic.
First, they did not need to consume blood anymore or keep themselves healthy. Second, Bela, Daniela, and Cassandra are no longer weak to the cold, meaning they won't die in the cold anymore. Third, if they die, they will respawn in their rooms at the castle. And last but not least, they are compelled to obey Izuku; after all, they are his quirk.
The one thing that does not match the fanfic is that Izuku is not a soft and shy femboy that likes being pegged by his quirk; instead, he is a tentacle monster, an eldritch abomination of unfathomable power, but oh well.
Anyway, three weeks into having his new quirk in, Izuku can say two things: first, that he really enjoys Alcina's company. She is an amazing woman, elegant, highly cultured, and able to have good conversation for days, and above all, she has impeccable taste in music and has a great singing voice.
The second is that her daughters are absolute gremlins and little shits that can and will get in trouble any chance they get.
The thing is, he is a tentacle monster living in a castle with a nine-foot-six MILF and her three daughters, which is fertile ground for all the dirty jokes you can imagine. The only reason things aren't more weird is because this isn't an Addams Family crossover fic, because an eldritch tentacle monster living in a castle with a giant vampire MILF and her three daughters made of bugs screams Addams Family.
Izuku is half expecting to wake up one day to Gomez knocking on the front door to invite him to a friendly fencing match.