He discovers he can cast a total of ten effective days' worth of spells before he gets a blinding headache, each first level spell taking him six seconds to re-memorize with the sharingan. At that point he needs to lie down until his head stops hurting, because ow, and even once the pain subsides trying to memorize more just causes another spike.

After practice he's ready to show Kakashi the shrine, if Kakashi's available?

He's got nothing else to do this evening.

Then Sugira shows him the hidden stairs, and the weird thing in the shrine, and relays what he concluded about it, through some odd combination of the sharingan and detection spells.

"I don't know if it'll sense if the opener is Uchiha."

Kakashi pulls up his headband and takes a look.

"It's meant to be a lock or gate of some kind. Unusual style... I think all it's looking for is the sharingan and the fire. The vast majority of the clan had a fire affinity, so that makes sense."

"Right. Safe to open, then?" Also fairly simplistic but it might be harder to blood-lock things here.

"Should be, as far as I can tell."

He nods, walks up, and traces the 'samsara' character with fire chakra gathered in his finger.

Father to son, mother to daughter

Herein lies our history

Claim your mastery

Heed these warnings

He peers closer at the stone tablet.

The writing, though some details are blurred, describes an ancient war, a tree that was the source of all chakra, and a woman who ate the fruit of the tree to save her people and became the first to use chakra. It tells that her son defeated the tree after it grew angry because of the loss of its fruit, and that he was the Sage of Six Paths. The Uchiha are descendants of that bloodline and inheritors of their power, manifested in the sharingan. The first stage of the sharingan is observation, insight, mimicry. As the first stage was awakened in need, the second comes forth in grief. This is the Mangekyou and it is power, unique to each. Use of the Mangekyou comes at a price; over time, it will lose its light. To have sight restored, another's Mangekyou must be taken.

Okay that sounds sufficiently narratively dramatic. He was wondering where the drama came in. There better not be a bloodline curse.

Also there is no way in fuck that story isn't becoming relevant someday. Creepy hidden tablet in significant location? Definitely a plot hook, and Inanna's not exactly prone to subtlety. He also, of course, doesn't trust it to be accurate - he's read enough history, compared his mother and other survivors of the Unification Wars' stories often enough to know memory's unreliable and stone's worse, and they have actual survivors from that time.

"Can you read that?" he asks Kakashi.

He approaches closer and peers at the stone.

If Sugira's paying close attention, he might catch a flash of movement across Kakashi's eye before he grunts in pain and slaps a hand over it. A drop of blood leaks out to splash on the floor.

He catches it.

That seems an awful lot like drama.

"Are you okay?"

"Nn. Might be." He slowly lowers his hand. There's a new design on his iris, a black pinwheel etched on the red background with three spokes and hooked tips. It's there for a second before melting away into the regular design of three tomoe in a circle. "I think I just activated the Mangekyou."

His team are all idiots who get themselves injured reading.

"Then you saw the warning?"

"Yep. Now I get to go blind. Also saw some neat stuff you could do with two of these, but, well."

"I think the blindness won't kick in if you don't use it. Any more historical information?"

"Mm, big war, first woman to use chakra, two sons, creation of the tailed beasts—that was interesting actually, the sharingan can be used to control them..."

"So more than what I can see." He rubs at his face. "I suspect that might end up relevant somehow..."

"I hope it doesn't. A lot of that didn't sound like very much fun."

"Still." He shrugs. ...He wants to ask if Kakashi is sure his eye's okay but also Kakashi is a grown ass adult and Sugira isn't, actually, his parent.

"I think we're probably done here?"

"Yeah."

"Thanks. For bringing me here."

How do feelings work.

"Ah - okay?" Pause. "Just... Try not to go blind."

"I'll do my best."

They leave the shrine, and Kakashi heads off.

He resets everything on their way out, as best he can.

His tasks over the next few days are mostly telling Chihiro about activating the sharingan, offering to return the book, and offering to show her the marks he found.

And, of course, trying to get Kakashi to sit down for arcana lessons, while keeping up with his chakra training, going through the archived Uchiha weapons to find chakra blades for Naruto (he finds dual kukri, close enough to Naruto's ideal), continuing spell development (made faster by the sharingan)...