it

What does 'you shall have it' mean? Is it a set expression?

“Many of our oldest family trees become a little diseased over
time,” he said as Bellatrix gazed at him, breathless and imploring.
“You must prune yours, must you not, to keep it healthy? Cut away
those parts that threaten the health of the rest.”
“Yes, my Lord,” whispered Bellatrix, and her eyes swam with tears
of gratitude again. “At the first chance!”
“You shall have it,” said Voldemort. “And in your family, so in
the world . . . we shall cut away the canker that infects us until only
those of the true blood remain. . . .”