It was either two fics or two chapters.
In the first one, post 'Avengers', Loki was taken back to Asgard, and by mistake locked in a dungeon that had already been ocupied - by a vampire. Apparently the creature had been locked up for so long, that nobody remembered it's there.
Naturally, Loki got attacked and turned, escaped, and came to Stark Tower looking for Tony's blood.
He got it.
Then something happened (don't remember what exactly) and Loki left in a hurry, leaving Tony injured but alive.
In the second one, Loki returned to Tony, but this time to talk, or possibly thank him? Because Tony's blood did something beneficial for him? I'm really vague on the details there.
Does that sound familiar to anyone? I tried searching for vampire!loki on ao3, and google for various word combinations, but got no results.
I'd appreciate any help, it wasn't a long fic, but the vauge memories of it are eating at my brain like a zombie. :(