I'm still sticking with shortish fics so this week's choice is a 6 parter from October 2008, This Other Darkness by metztlimoon . It's rated NC-17 and looks to be fairly dark....
Summary: When Ianto begins acting strangely, craving blood, becoming violent, Jack must enter Cardiff's underworld to find a solution. But does Ianto want to be saved?
Warnings: Blood, vampirism, mind control leading to iffy consent.
Ooooooh, creepy! Think I'll hide under the bed for this one!
Part one is here Parts are helpfully linked.
Happy reading! Join me under the bed.