Severus stalked into Dumbledore's office without so much as knocking, his mind still fresh with the scenes he had accidentally gleaned from Potter's Occlumency lesson that evening. Some of which were too damn close to home for comfort.
"Pardon me, Albus," he said as he passed the headmaster, "but I need to use your fireplace."
"Out visiting so late?" Dumbledore raised an eyebrow at him, but Severus had practically invented the gesture, so he felt no obligation to divulge all the details.
"A small matter needs taking care of - I'll be back through shortly." He continued on through to the sitting room and helped himself to a pinch of the Floo powder, sticking the rest of the jar in a pocket for the return trip. The Dursleys were sure not to have any. "4 Privet Drive," he intoned quietly enough for Dumbledore not to hear, and stepped into the fire.
Vernon Dursley was an oversized muggle man with both an enormous gut and an enormous ego. Severus had seen as much from Harry's mind, of course, but somehow the actual man was somewhat . . . less, somehow. He had expected him to at least be intimidating, but upon seeing a fully-grown wizard stepping out of his fireplace, the muggle simply jumped to his feet and gibbered.
"What's happening, dear?" came a voice from the hallway, and then Severus came face-to-face with Lily Evans' sister Petunia. The years had not been kind - she was bony and angular in all the ways her husband was not, although she was no more enthused about a wizard in her home than he was about being there.
"I suppose you're one of those freaks from Harry's school, then?" she demanded in a breathless voice.
Severus drew himself up to his full height. "I am, indeed, one of Harry's instructors. I'm also a Death Eater, if that gives you any insight into the sincerity of what I am about to say."
Petunia paled. Severus smiled coldly. "I see you recognize the name - yes, Death Eaters are who killed your sister. You understand what I'm capable of if you don't do exactly as I say."
Vernon Dursley gibbered louder.
Severus quickly withdrew a shimmering vial of potion from his robe and splashed it directly in the man's face. Dursley sputtered, clearly outraged, but Severus was already past him and pouring the rest of the potion over his wife's head. It was a shame, really, that he couldn't just use a spell, but it was dangerous enough just using the Floo Network - leaving a magical signature would have definitely alerted the Dark Lord that something unusual was happening at this muggle household. And Severus did happen to have the potion handy, easy to grab on his way from his office to Dumbledore's -
The noises coming from Vernon Dursley's mouth seemed to have mutated into speech at last. "-don't know what you think you're doing, coming in here and threatening us like this," he was whining, "but we don't want any of your sodding wizard wa-"
Severus reached out and grabbed the man by the throat, effectively silencing him. He inched the man's jawbone up higher, until Dursley was forced to look him full in the eye.
"You may notice your skin burns right about now," Severus said in a conversational tone. "It will only get worse the longer I touch you. If I keep my hand here long enough, it will leave a blister on your neck. And longer than that, it will cause burns your muggle hospitals can't treat. I'm sure you'll be thrilled to discover that any contact with any other person's skin - including your wife's - will produce the same reaction. And it is cumulative between reactions."
Both Dursleys paled, but Severus wasn't done yet.
"I'm sure you're not lamenting the loss of your intimate life, because I'm assuming you don't have one. But if I hear one word against you from your nephew - one tiny insinuation that his next summer with you isn't the most magical experience of his life - I will not only let him in on our little secret, I will also find your son Dudley and share some of my special potion with him. His friends at his muggle school will certainly find the effects interesting, wouldn't you say?"
Vernon Dursley was gasping for air now, alternating between trying to pull Severus's hands away from his throat and then letting go as the contact with Severus's wrist burned his palms. Severus gave one final squeeze for good measure, then dropped his hands and turned on Lily's sister. "You understand, I hope?"
She nodded, too frightened to speak, which pleased Severus immensely. Both Dursleys were rooted to the spot, following Severus's every movement with wide eyes.
"Since I see I have made myself clear, I will leave you to your evening. Mr. Dursley, Mrs. Dursley, adieu." He turned on his heel (with perhaps more dramatic flourish than was strictly necessary) and returned to Dumbledore's office.
"I trust your social call was uplifting?" Dumbledore inquired mildly as Severus stomped back through his office.
"Informative, I'd say," Severus replied. "I rather find myself looking forward to another visit sometime in the future."