Transfusion
The new prisoner seems unalarmed, but that will change soon enough. The metal barred door clangs shut. The cell's other occupant doesn't move.
"Vampire." The prisoner's lip curls up slightly. "You look like you haven't eaten in weeks."
"Since I was captured." The monster's voice is rough with disuse. "They intend me to bleed you dry."
"Well then. It would not do to disappoint them."
The creature moves in a blur; blood spatters the bars.
Only minutes later the captors realize their mistake. Far worse than one half-dead vampire are a fully fed monster and a man who cannot die.
Written as Joe and Nicky from The Old Guard, but this could honestly be read as any vampire and other immortal that you like. (I'm also tagging for the Dreamling folks because this feels consistent with that ship dynamic.)
I've been toying with writing a longer fic in which Nicky is a vampire, although either of them could be the vamp here. If I do, I'll probably expand on this scene, which was originally a tad longer and edited down to drabble length.
I'm such a slow writer that I'm posting all my @monsterfucktoberbingo fills at the end of the month. Sorry mods!