"Yeah," Snape replied, abstracted, breathless, eager. His eyes glittered, so black they seemed to be made entirely of pupils blown wide by arcane excitement. He stirred the potion, lifted the crystal stirring rod from the furiously boiling cauldron, and licked the hot mixture from the rod.
Swallowed, licked his lips, as his stare grew hooded, abstracted. "Yessss," he hissed vicious triumph, exhaling a plume of sickly green smoke.
Then he seized Selwynn's wrist in one hand, hooked up the cauldron's wire handle with his other hand, and upended the whole boiling mass over Selwynn's forearm.
As the mix seethed and sizzled, he chanted in tones thick, choked, somehow lustful: "Pain for surcease from pain, burning for balm from burning, sacrifice to sate the hunger for sacrifice... Fiat voluntas mea!" He stabbed his wandtip between the eyes of the skull, and the entirety of the potion disappeared in a miasmatic cloud.