Joker/OC story. Not a dark fic, just what I think it could be like if he were to ever fall in love, or as close to love as the Joker could get. Full story can be found under RubyWaters at fanfiction.
The dark sequel of The Laughing Man series. Harley Quinn is injured and the Joker leaves her in the care of an old friend. But without the controlling hand of her lover, Harley begins to see her life in a different way.
A dark psychological tale of Harley Quinn's origin. It begins in Arkham Asylum after Dr. Harleen Quinzel is assigned the Joker's case and desperate to make a connection to her mad patient, she starts a game between them that will forever change her life.