Itachi doesn’t remember the first two years of his second-life with any degree of clarity.
No, really. He doesn’t. It’s a peculiar phenomenon to the former-Uchiha now-Black child. His sense of self-identity is rock-solid. He has known who he is from the moment he breathed his second first breath.
There are many ways a life can be tragic. Not many are quite as comprehensively terrible as his own. He lived - he died - A wretched finale of a wretched life. Death - Peace - Quiet -
Not for long.
Kabuto tears his soul from death into non-death, hammers it into an unnatural, un-living, un-worthy container.
And then - instead of death - birth. From one moment to the next. A not-dead corpse -- blink — a newborn. Wet and cold and shaking with biologically imperative terror.
Terminal depression hits more or less instantly. There is nothing for Itachi here, in this ill-fitting world, alien on a cellular level. Whatever beings populate this world, they wield powers beyond his understanding. They keep his body alive through unknowable means. Judging by their panicked voices and ever-increasing numbers, this feat is not easily accomplished. He doesn’t give it much thought.
Whatever their methods, they succeed in a feat of medical engineering. These mysterious beings edit, sculpt and adjust, they prod and yank and change until the body (his, Itachi's body, Amaterasu preserve him) is capable of sustaining the complexity of his soul. Of his thoughts, to be more precise. A miracle, one might say. A curse is what Itachi knows to be true. He didn’t ask to live. He desperately does not want to live. He spent a decade wishing for death, praying for eternal nothingness. And yet - here he is. Alive and workably functional.
His - what? Parents? Soul-kidnappers? Unwitting wardens? The humans in charge of his life are entirely unimportant, which doesn’t spare Itachi some osmotic awareness of them. Physically they are indistinguishable from any other adult-type-human, in that Itachi’s baby eyes can’t distinguish them. Not that he cares to look. He can’t even conceptualize having enough willpower to voluntarily look at something.
Their main distinguishing characteristic is how reserved they are. They are likely hedging their bets, holding their affection back until the strange infant’s fate is clearer. Alternatively, they could simply be phenomenally cold people - not unlike some Uchiha he could name. Their reasons are, of course, entirely immaterial. One way or another, aside from the medics, Itachi’s only caretaker is a being so overwhelmingly supernatural, that it’s almost enough to shock him out of his willful amble towards death.
The creature loves him with everything it is. It’s also a slave of some kind - which is precisely none of Itachi’s business. It fusses over him endlessly, rasps various lullabies that he has no hope of understanding, but that he suspects are thoroughly blood-soaked and gruesome. It is, perhaps most importantly, the only being, human or otherwise, willing to hold Itachi’s strange little body. It’s the creature that rocks him, pets him, burps him. Its leathery limbs provide warmth, comfort and security - all those things his baby-brain is wired to respond to.
He could, in truth, be its actual child. So far, Itachi’s limbs feel smooth and pink, but who even knows - perhaps this is what he will age into? Well. What he would have aged into, in the unlikely situation he survives past the day he is physically able to hold a blade.
In a sense, the strong vein of apathy dominating the depressive episode is to his benefit. He can’t muster up the will to actively end his misery. His wrinkly, fantastical caretaker is an impeccable watcher. He will intervene with whatever mystical force he has at his disposal at the first sign of visible damage to his charge. If Itachi ever manages to scrounge up the willpower to escape this strange un-life, he will have to take swift and decisive measures to ensure he stays dead. Who knows, maybe he might be granted the reprieve that time?
Drifting is clearly the only path forward. So he works on getting a firm handle on this new body. Slowly manages to manipulate his heartrate, slow it down, dim the world until only the eternal blackness of his mind is left. There is no space for despair there, or rage, fear or sorrow. There is nothing at all, the closest approximation of death his mind can build.
Regulus Black breathes his first two years into Itachi’s go at dying by sheer force of will. Or lack of will, depending on the viewpoint.
It changes - everything.