Who was Peter Pettigrew?
A friend of James Potter and a member of the Marauders, he had secretly surrendered to Voldemort after graduation and become a Death Eater.
He had betrayed the Potter family's whereabouts, leading to their deaths, faked his own death, and framed Sirius Black. The Ministry of Magic had even awarded him the Order of Merlin, First Class.
In reality, Peter Pettigrew's animagus form was a rat. He had infiltrated the Weasley family and stayed with them for a full twelve years. Will naturally knew all about this.
But he had another use in mind for this rat. What fun would there be in taking Scabbers from Ron and forcing him to submit to observation and experimentation?
Then he would have to resort to violence. All that fighting and killing was such a chore. Will was a good man, after all, and he had already thought of a way to get the best of both worlds.
In the Room of Requirement, Will began writing a letter.
He opened with "Dear Professor Riddle."
In the letter, Will stated that he had already entered the Chamber of Secrets and begun preparing to deal with Harry Potter. The details were still being arranged; he intended to lie low for a while, then strike a fatal blow.
He was also preparing to become an animagus and hoped to obtain a test subject.
He had discovered Peter Pettigrew and hoped Professor Riddle could order Pettigrew to obey him. Peter's current identity was particularly useful—he was Ron's pet, which made it easy for him to provide Will with information.
"Dear Professor Riddle, I await your return at Hogwarts."
He signed it, "Your student, Will Behrens."
That was right. Will planned to use Voldemort to order Peter around directly. Experiments went much better