"-- the composition of waters, movement, growth, embodying and disembodying, drawing the spirits from bodies and binding the spirits within bodies" -- Zosimos of Panopolis
It looked like the cauldron was full of dull, murky glass -- flat grey and streaked through with marbled veins of white. Draco almost expected it to chime when Snape touched the knife to it, but instead the blade sank in with a hissing grind, as though through sand or lead.
"How am I supposed to get Potter to drink that?" He asked, watching Snape cut loose a rough, lumpy cone from the mass in the cauldron.
"Drink it?" Snape shook his head, lay a hand between Draco's shoulders and pressed him down across his father's desk. "Don't be an idiot," he said, fingering Draco's loosened, slick hole with his free hand. "No one could drink the potion at this stage. Now relax."
"Again?" Draco complained more from reflex than reluctance as he thrust back against the practiced invasion, "Are you sure you can get it up this soon aft-OW!" He glared over his shoulder, buttocks smarting from the slap. "That was uncalled for!"
"On the contrary, my little athanor," Snape purred, holding up the chunk of potion, which, now that Draco looked at it, was half as long, and about as big around as Snape's... Draco swallowed. "Entirely warranted," Snape continued, nudging the mass against Draco's anus, "Now be still..."
Draco groaned as the hard but not hard, smooth but lumpy, warm and cool and tingling-with-magic plug pushed in past the ring of muscle his recent orgasm had so relaxed. Snape's come and the lube they'd used eased the passage even more, and Draco couldn't help gasping and writhing back in a stretch as the plug nosed past his prostate. Then it pushed in further -- ALL the way in, and his anus closed around Snape's driving finger as if in surprise. "Wait!" Draco gasped, clutching Lucius' desk in alarm, but Snape just pushed the plug in as far as his finger could reach, and Draco's startled clench only drew it further in. "Wait, how will I get it out again?"
Snape drew him upright, held him against the combined urge to stretch in delight, and to squirm in horror as the plug shifted. Then he smiled a kiss against Draco's sweaty temple. "You won't. Potter will. And then he'll be yours as you demanded, little tyrant."
Draco shudder-sighed, leaned back into his protector's arms. "For a little while, right?" He could feel Snape's face harden, and nuzzled back, stroking his hair across Snape's hollowed cheek and cajoling softly. "Just a little while, if all goes well, and then..."
"And then, assuming he or I or you do not go mad... Then we will both be his." Snape pushed Draco away suddenly, swooping across the library to collect his scattered robes. "You ought to get dressed. We mustn't be gone from Hogwarts too long."