Was it Voldemorts soul?

[choki]

Hmmm... I thought the theory was Harry was the horcrux?

He intended to make the last horcrux (7 was the magical number) after the killing of the Potters to celebrate the destruction of the prophecy. When the killing curse rebounded, Voldemort's soul (highly unstable due to him splitting his soul several times) split violently and a part of it hatched itself onto the nearest living thing - Harry Potter.

[salemboy]

Either way, he actually could have achieved magic number seven if he had merely not made Nagini a horcrux, which probably WOULD have made him invincible, because of the magical properties of number seven.