As part of a quest to un-fuck Boccob's Blessed Book that a group of Vecnites had scrambled during a ritual to convert the book to their deity, my adventuring group had to inscribe a dark secret that had never been revealed to anyone before.

We immediately started coming up with in-character secrets, some of which actually related to our backstories, others we just made up on the spot.

My character's in-story reason for why a new sibling always showed up when the previous one died was a family curse. It gave each member of the family an awareness of when other members were in grave danger - which, being adventurers, means a constant sense of impending doom hovering over them at all times. When one of them is about to die, however, the feeling gets significantly worse, and they try to reach their scattered siblings to help... but never seem to show up in time.

I had my dark secret be the actual reason why the family had been cursed - something only known to the bloodline.

(I'd tried having the secret be why the youngest member of the family (a battle-sorcerer) had been kicked out of the army, but evidently that story had been passed around taverns too many times to count.)

The psion came up with the story of how they became an Elan (a ritual done in secret), the barbarian was going to inscribe how the story of how they first raged (no living witnesses), the bard figured she'd had a Speak with Dead conversation with a long-dead king she'd found in a mass grave deep in the Underdark, and the ranger said she'd used Speak with Animals to learn secrets from an archmage's familiar, still alive after the archmage had passed.

All of this came out of a ten minute conversation while we all tried to figure out what terrible secret we could inscribe into the book to kick-start the recovery process, and was a lot of fun.

What's your best in-character deep, dark secret?