"You say all this..." Voldemort's voice trembled slightly. "What does it have to do with Harry Potter?"
"Just as I said, the Cruciatus Curse evolved from the Killing Curse," Vizet said. "It was actually an attempt to recreate, by some means, the Killing Curse's effect of harvesting souls."
"If the Killing Curse is a sharp blade, capable of neatly separating blood, body, and soul, then the Cruciatus Curse can only be considered some kind of malformed weapon. It can only tear the soul apart, with almost no cutting effect."
"Because the Killing Curse is clean enough, once the soul is separated from the blood and body, one meets death before even having time to feel pain."
"The Cruciatus Curse is another matter. It can only tear the soul apart, which is why it brings indescribable pain—the effect everyone knows so well."
"As for the connection between the Killing Curse and Harry, it naturally began when you struck Harry with the Killing Curse all those years ago, and under Love's Magic Protection, the spell rebounded onto you."
"The Killing Curse still took effect. It tried to sever your soul from your body, but you had already created multiple Horcruxes, leaving your soul extremely unstable. That was why such a special situation arose."
"Because of the Horcruxes, your soul was anchored and avoided being swept into the 'vortex representing death,' but it could not avoid being affected by it, causing a Soul Fragment to split away."
"Because the Killing Curse took effect, the connection between your soul and body had already been forcibly shaken. In a sense, at that moment, you truly stepped to the brink of death."
"And that torn-off Soul Fragment could only choose to attach itself to a suitable living creature nearby. So it had only one possible target..."