Albus Potter and the last Horcrux.

The Theory

McGonagall had a sorrow look in her eyes, she started to explain all. "There once was a boy who made all the wrong choices. Tom Riddle. More commonly known as Voldemort. He, as you may know, made seven horcruxes including your father, Albus. He wasn't a very good boss either. From what the Ministry says, Voldemort can no longer come of power, yet my last words with Professor Snape said different. He told me that Voldemort made another Horcrux by accident. The story is that one time in the Whomping Willow Tom got very angry at Peter (Wormtail) and casted the killing curse at the inside of the tree. That part of the tree grew and died like all beings but one piece of the dead bark still has the smallest fragment of Voldemort's soul in it since the killing curse with a fragment of his soul just clasped itself onto the nearest living thing it could find.The tree."
"What has it got to do with us?" Asked Carlos
"Well, Carlos, to get this willow to stop trying to kill you, you must touch the knot. I have watched over you and I have a feeling that you boys can destroy it with a basilisk fang that Professor Dumbledore kept." was the answer.
Carlos mumbled in agreement.
"The only thing is," McGonagall gravely started,"there has somehow been a curse on the tree, only animals can touch the knot. Humans are repelled."
"How can we do it then?" Asked Louis
"She thinks we are good enough to become animagi you dumbo." Carlos told him.
"Now you will have to figure this yourself, boys.I, as you may know, am an animagus but you still need speed and stamina. Which I do not." said McGonagall as she swept away.

The four went back to the common room and once again sleep flew in as quick as a bird.