The Dementor's intangible outer form was like a shattered porcelain doll, bursting with countless tiny cracks.
Blue light shone through the cracks—the light of the Patronus Charm.
"Ugh—" The Dementor let out a grating shriek.
Then, its form exploded into a cloud of black mist.
When the black mist dispersed, only a mass of solid black substance remained in Matthew's hand.
"So this was the Dementor's original form?" Matthew could sense that the Dementor had not died.
When he dismissed the Patronus Charm, the black mass began to emit black mist. After a while, the Dementor's intangible form condensed once more, though this Dementor's form seemed somewhat smaller than before.
Matthew repeated the same process twice more. He found that not only had the Dementor shrunk by a full circle, but it also took longer to condense its form, looking rather listless.
"If I repeat this a few more times, will this Dementor starve to death?"
Matthew stopped experimenting. He did not want to kill this Dementor yet.
After sensing it for more than an hour, Matthew had gained a solid grasp of the structure of the Dementor's physical core.
He had already fulfilled one of the two prerequisites for practicing Black's transformation: understanding the bodily structure of a Magical Creature. Now, he only needed to refine the potion.
He could refine potions in the Room of Requirement, and he had prepared the materials long ago.
"Mandrake leaves... Death's-head Hawkmoth pupae... Fluxweed picked under a full moon... Lacewing Flies... dew... African Tree Snake skin..." Matthew tossed the Potion Ingredients into the cauldron one by one. Then he gripped the Dementor's physical core again and dispersed its form with the Patronus Charm. "And finally, the Dementor's core."
Half an hour later, Matthew had a bottle of pitch-black potion in his hand.