Harry could not be told before or after they acted. He was Voldemort's living Horcrux, and under certain circumstances, their minds could briefly connect. Neither Dumbledore nor Will would tell him.
Will would strike in secret and "kill" Harry without revealing his identity. Of course, he would do it with a "Basilisk." The real Basilisk was already dead, and Dumbledore had even divided up its body, giving Will a share of the spoils.
Snape still had a few Basilisk parts to use as Potions ingredients.
The "Basilisk" Will would control would be one Dumbledore had created through Transfiguration.
It would happen soon. Will would act in the coming days, as the old bee—no, Dumbledore—wanted. Once it was done, Dumbledore would teach Will Transfiguration.
For Will, that was an opportunity too.
"With Pettigrew here, I understand changes in form even better. Give me a little more time to raise Transfiguration to LV5, and becoming an Animagus won't be a problem. I might even gain a new transformation."
Thanks to Sight Insight, Transfiguration's Bonus Ability, Will could use Wizard Sight to see Peter Pettigrew's magic pathways directly. His understanding of the Animagus grew clearer by the day.
"Too bad I can't get my hands on Heavenly Stems Transformation, the book mentioned in Primordial Spirit Beast Explained that records transformations into wondrous beasts. Otherwise, a Magical Animagus might not be out of reach."
There was also that passage: "A Primordial Spirit Beast of the bird kind may turn crimson from head to tail, breathe flames, and deceive the world with False Phoenix Form; a Primordial Spirit Beast of the python kind may grow horns and extra limbs, soar through clouds and mist, and take to the skies like a Jiaolong."
Will did not want an imitation. He wanted the real thing.
"Potter!