Midoriya Izuku has a voice in his head.
At first, Midoriya thinks it is a Quirk. Sentient ones aren't unheard of, after all, though they are a tad bit rarer than most other types. However, after months of the voice only saying "Go West," he starts to wonder. Inko takes him to a series of doctors, and genetic profiling shows a lack of markers present in all people with Quirks.
Midoriya is Quirkless.
From there, it's a trip to a psychiatrist, who is also puzzled. Schizophrenia tends to be more pronounced in its effect on the person, and Multiple Personality Disorder is much more distinct. Medical doctors have concluded that there are no abnormalities in Midoriya's physical brain, and it isn't a Quirk, meaning the voice is purely a mental construct.
Nobody is quite sure what to make of it. For now, the best that the psychiatrist can do is prescribe some sort of medication that should quiet the Voice, as it's been gradually getting louder as the weeks go by, to the point of giving Midoriya headaches.
The Voice doesn't like that. It begins screaming inside his head- a loud, guttural, desperate type of screaming, and Midoriya's only 4, halfway to 5, so he starts freaking out as well. Very concerned, Inko and the psychiatrist do their best to restrain Midoriya, even going as far as sedating him with an anesthesia kept for situations like this.
But then, there is fire.
The psychiatric hospital burns down. Several people die. Inko is one of them.
Midoriya Izuku's corpse is not found.
Go West.
Dazed and half-mad with grief, barely comprehending what he'd just done, the fact that he murdered his own mother, Midoriya complies.
Midoriya is six now, halfway through China. He stowed away on a boat from Japan heading to the Asian mainland, and has slowly been working his way across the country on his tiny legs.
Food is hard to come by. He has to beg for scraps. Sometimes he has to steal. It's not what a Hero would do.
It keeps him alive, though.
He continues West.
India, Pakistan, Iran, Iraq, Saudi Arabia, Jordan, Israel.
Midoriya is ten, and the fire in his veins is burning hotter and hotter. It's difficult to hold it back; it wants to devour everything around him to fuel itself. He knows, though, that if he loses control, he won't get it back. Not until the inferno has had its fill.
He finally arrives in Egypt, and something inside him shifts. He knows he's close now.
He walks barefoot through the desert- his shoes wore out years ago- but the relentless heat of the sun barely bothers him.
Midoriya reaches the Great Pyramid of Giza, and the Voice finally says something other than Go West.
Burn, it commands.
He does.
Midoriya wakes up to a changed world. The sand he marched across is now glass, as far as his eye can see. The crescent moon hangs above him, tilted at a strange angle, unmoving. It smiles down at him with a demented grin. The sky is black, not a single star in sight. and there, crackling on top of the Great Pyramid, is the most brilliant fire he's ever seen. A vivid orange, brighter than the sun. A comforting warmth washes over him.
The First Flame, the Voice whispers reverently. The Age of Dark has been pushed back. For now.
Around the world, people are losing their Quirks by the millions.
Tidal waves and tsunamis stir the ocean, as those dwelling in the depths rise to the surface.
Earthquakes shatter mountains, releasing ancient evils from stone coffins.
Dreams are warped into horrendous nightmares, as the last safe haven, sleep, becomes a realm of torment.
The Old Ones have awoken, and they are not happy.