Author's Note: This contains spoilers for pretty much everything with Itachi, or at least it won't make sense without knowing about that. And so no one gets mad, I'm going to warn that there's implied Sasuke/Naruto (or the reverse, if you prefer) in that they are lying together at night and also engage in physical contact for reasons other than beating each other up.
"Kakashi-sensei told me about the Mangekyou Sharingan," Naruto says one night, apropos of nothing. Sasuke's hand is pillowed on his chest so Naruto can't see his expression, but he feels him flinch, as though even the sound of the word hurts him.
"What about it?" Sasuke asks, trying for casual but falling short at strained.
"Umm...what it is, what he saw." Naruto talks too quickly, obviously nervous. "I know it's none of my business, but I wanted to understand."
"You never asked me," Sasuke says. It's not a criticism so much as a question. Naruto doesn't say anything, but they both know the answer is 'Because I was afraid.' "It's different for different people," Sasuke explains. "It's up to the user to decide what they see."
"Oh," Naruto responds, voice small and uncomfortable in a way his so rarely is. The silence stretches out awkwardly between them, full of unasked questions and unshared secrets.
Sasuke breaks it first. "He showed me how he killed everyone," he says, in that flat voice reserved for horrors to great for any emotion to cover. "Both times."
"Both times?" Naruto echoes.
"Right after he did it," Sasuke clarifies in the same low, dead tone. "And again the time you saw. Over and over again." He puts a hand over his eyes and Naruto thinks for a moment that he's crying, but he's too still. It's as though he's trying to block out the remembered images, and Naruto feels terrible for bringing it up.
"I didn't know," Naruto says weakly. He reaches down and awkwardly places a hand on Sasuke's shoulder, trying to offer some small measure of comfort without knowing how.
There is a long, tense pause before Sasuke reaches up and takes hold of Naruto's hand. "I never told anyone," he says softly.