the_debt_of_time

Sirius shifted back to human form and pointed his wand directly at Snape's throat.
"Snivellus."
"Black," Snape's voice dripped with scorn, not looking surprised to see Sirius. "I'd
heard rumours of your unfortunate return from the grave. Not that you were actually
buried, of course. Tell me, were you even dead, or was it another poorly planned escape of
yours?"
Sirius growled menacingly. "Get back up the stairs."
Snape sneered at him in reply, black eyes narrowing. "I advise you to get your wand
out of my throat, dog. I stopped taking your threats earnestly years ago."
"You really shouldn't have. Get up the stairs now, Snape. I happen to have need of
your Floo."
Snape's black eyes met Sirius's steely stare, neither man willing to back down. "And
why would I possibly grant access to you?"
"Because your only hope of surviving another encounter with me hangs on the fact
that a certain young, blond Death Eater has vouched for you, and I need that fucking Floo
to speak to him." Sirius waited for a reply, staring coldly at his lifelong rival.
If Snape was surprised by the revelation, he did not let Sirius know it. "And how do
I know that young Malfoy is, in fact, with you?"
"Because unlike you piece of shit Death Eaters," Sirius snarled, "I don't kill
children."
Snape moved only slightly as he turned to the statue. "Speaking of children, I can
only assume by your lack of emotional breakdown that Potter still lives? Dumbledore," he
said, and the gargoyle slid aside, revealing once again the large spiral staircase.
"Dumbledore is your password? You're sick."
"And you are as dense as you ever were, Black," Snape said as he climbed the
staircase. "If, in fact, Mr Malfoy has informed you of my allegiances, then you should know
the headmaster's death was, in truth―"
"I don't want to hear it," Sirius snapped. "Just get me to the bloody Floo!"
Snape glowered as they entered his office. "Very well."
"Ah, see? I knew my worthless great-great-grandson wasn't really dead. Hiding out
was he, Severus?" the portrait of Phineas Nigellus Black remarked, staring daggers down
at Sirius.