"I cannot ssstand this form for everyday ussse, Ssseverusss," Lord Voldemort hissed, the sibilant sounds echoing strangely throughout his (not-so-) secret lair. "That ussselessss lump Wormtail obviousssly made an error in the potion. And thisss damnable ssspeech impediment! I sssound like I'm ssspeaking in parseltongue half the time! I nearly frightened the houssse-elvesss to death when I asssked for tea. I never realisssed how many Ssss I usssed in common conversssation until this debacle. Thisss isss unacceptable."
Severus fought something that, on a more pleasant man, might have been called a grin. Lord Voldemort's eyes narrowed dangerously.
"Are you sure you wish to change your appearance permanently, my lord?" the potions master asked cautiously. "The potion can have unexpected outcomes, and glamours could function perfectly well as an alternative." Besides, Severus thought privately, your new form is very intimidating, and that can only be good for a Dark Lord. Unfortunately for him, that thought was not private enough to escape the very skilled Legilimens which was the Dark Lord.
"Did you not consssider my original form intimidating, Ssseverusss?" Voldemort hissed in a nearly pleasant tone that screamed danger in loud, flashing colours to anyone who had even once glanced at the Dark Lord's shadow for a split second.
"I believe you are intimidating in any form, my lord," Severus said delicately. "However, many more would be terrified by your very presence now than when you had your original form." Someone who believed the Dark Lord's charming front, for instance. That group, however, was mostly made up of imbeciles and Hufflepuffs, who were, Severus admitted, practically interchangeable.
"Hmm," the Dark Lord considered. "No. Perhapsss I shall ussse thisss visssage asss a glamour on raidsss, but thisss will not continue. The potion, Ssseverusss, and sssoon."
Dear Merlin, Severus thought, No wonder he's getting irritated. I would be as well if I spoke like that.
"Your potion, my lord," Severus bowed, carefully keeping the potion steady as he did so. Voldemort frowned disapprovingly.
"You sssmell like rodent blood, Ssseverusss," he pointed out. Severus wondered briefly how the Dark Lord could smell blood well enough to differentiate the smells of the blood of different species, and why he knew what rodent blood smelled like. Voldemort stared at Severus pointedly, waiting for the potions master to recognise the snake-like face he wore and also the snake draped around his shoulders, which had just whispered to him how Severus smelled. Severus did not get the hint literally glaring at him from the throne and instead proceeded to explain the rodent blood.
"My apologies for the smell, my lord," the one I don't smell myself and my nose is rather close to the rest of me, how did you smell that?! "but I, hmm, tripped over, let us say, a rather portly rat named Wormtail in the hallway earlier. I merely set a rat trap." A very, very large one. Wormtail had been in human form at the time.
Voldemort gave an strange smirk which was oddly similar to a poorly concealed grin. "Very good, Ssseverusss. Now, the potion?"
"Of course, my lord," Ssseverusss– whoops, Severus– replied, and proffered the vial carefully. Voldemort grinned fully this time and downed in a matter of seconds as Severus gaped.
If Severus hadn't been there, Voldemort thought unhappily, he would've been screaming in pain. As it was, he had to suffer in silence, which only made the ordeal even less pleasant than it could have been. However, it had to end at some point, he knew– and it did.
"Mirror," Tom Riddle said sharply, still unhappy about the pain he had just endured. Severus conjured one quickly and backed away a good few steps, allowing his master to examine himself in peace.
"It seems to have worked perfectly well," the potions master offered after a few rather dull minutes of watching the Dark Lord study his own face in a mirror. Without tearing his eyes away, Voldemort shook his head.
"Not quite," Tom Riddle said. "Although you are to be commended, Severus, this is a wonderful recovery. However, not quite perfect, as I said, because if you look closer, my eyes are still red."
But who would want to be that close to you, my lord? Severus asked in his mind, and proceeded to have a minor panic attack that he was now calling him 'my lord' even in his own Occluded mind where he was mocking the Dark Lord mercilessly. "Do you want me to attempt another dose, my lord?" he asked hesitantly, unwilling to make another batch without a good reason, due to the gargantuan task of scrounging he had had to take on for the last dose.
"No," Tom said, smiling almost innocently. It was very disturbing. "No, that's all right. I rather like them, actually. Very striking."
"Striking?" Severus repeated dumbly. What?
Harry Potter fell off his bed at Number 4, Privet Drive and began laughing. He didn't manage to stop until his uncle shouted at him to keep down the racket in there, boy. Harry never was quite able to explain to his friends why he and Snape exchanged amused glances (and hastily stifled giggles, though Severus had his students sufficiently trained to immediately repress any horrifying memories of him smiling– or indeed looking vaguely pleasant in any way) whenever they crossed paths for the first month of school that year.