I'm being daring with a longer fic this week - prologue and 11 parts. Chapters are fairly short though, so it should still be manageable, even for me.
Requiem is by seether_79 and is from way back in March 2008.
Summary: “I have this dream” Ianto begun as he looked up at the woman sitting in front of him. “I’m not sure what it is… memories from a life forgotten; or just me going crazy.”
Sounds mysterious! There are spoilers for the first two seasons and according to the author, it goes AU around the mid-point of the story.