Itachi had never been a skilled ninja. He couldn't throw a kunai at a target straight. He couldn't make a clone. The very idea of getting in a fight with someone was petrifying to Itachi. His Sharingan had bloomed into existence during a panic attack when his team had been interrupted on a basic D-rank mission to accompany the Daimo on a purely ceremonial trip. Real ANBU had been guarding the trail. They were meant to fight, not Itachi. But instead, his team had been killed and Itachi had been too scared to move, somehow phasing into the background and surviving the ordeal, now with two Sharingan in tow.
His father had been ecstatic to learn Itachi had activated the dojutsu, praised Itachi's ability to activate it for long periods of time and to mature it in a handful of months. The reality being that the Sharingan blossomed with Itachi's fright, and since it was his primary emotion, the Sharingan saw a lot of use, tomoe maturing even though Itachi had yet to use any of the new abilities his father reeled off.
Shisui of course was in utter stitches. At the academy the older boy had often shown up and demonstrated to Itachi how to throw weapons for target practise, then zipped away without retrieving his weapons from Itachi's target. As Itachi's kunai and shuriken had flown so far off course as to not be in the training grounds any more, the academy instructors had naturally assumed Itachi had thrown the weapons perfectly, believing Shisui's work to be his.
"I don't even know what Father means by the Eye Of Hypnotism," Itachi lamented one afternoon, watching Shisui dance around the training area, kunai flying gracefully from his fingers occasionally crashing into equally well thrown shuriken to ricochet into hidden targets.
"It's part of how the Sharingan forms genjutsu," Shisui explained, "Have you made any progress with genjutsu?"
"No, but I am fairly convinced Father is having a psychotic break." Itachi sighed, "He thinks I'm forming some kind of illusion where death thrums in the ears of those affected. I didn't realise he was able to hear how loud my heart was beating."
Shisui laughed. "That's bonkers. Ooh! We should give it a name!"
"What? No! I need to come out with the truth!"
"How does 'Death Call' sound? Or 'The Itachi Engine'?"
"Shisui! Be serious!"
But Shisui's joke had taken root. The Song of Death had become Itachi's signature 'jutsu'. The steady loud thump of Itachi's heart seemed to scare Itachi's opponents just an iota more than he was petrified of them. This lead to spontaneous fleeing of enemies, weaker ones outright dying from being frozen stiff by Itachi as though in a powerful genjutsu then taken out by his comrades.
It had been even worse when ANBU black ops killed his clan and Danzo placed the blame squarely on Itachi. Fleeing the village, he had ended up with Akatsuki, a band of S-rank criminals that were clearly insane. They travelled in two's, which meant Itachi would be mostly on his own trying to handle squads of high levelled ninja trying to kill him. The Song of Death generally didn't work against his assassins, though luckily Kisame, the man he had been paired with, was far more competent than Itachi and killed the ninja with a few flicks of his wrist, Kisame's sword Samehada tearing them to bloody ribbons.
When he was first rescued, Itachi had been entirely certain he was going to die. He stared up with wide eyes at the shark man as Kisame looked between the dead ninja and Itachi, who was shaking with fear as he cried on the floor, kunai still firmly in his pouch and a growing dark spot on his trousers from pissing himself. Unlike everyone else Kisame worked it out. And then rather than yell at Itachi or kill him for the lie, Kisame laughed and laughed until he passed out from lack of oxygen.