D&D 5E (2014) Bargaining with the Fates

[MarkB]

Another option is the Total Recall scenario, where their missing memories reveal them as having been someone they don't want to be.

 

[Kobold Stew]

Do your players respect the bonds/flaws/ideals on their sheets? If so, maybe the offer is a very generous one -- the fates are happy to restore your friends' memories with the fulfillment of a quest (taking the Cloak of invisibility from some level-appropriate challenge). But when they do, the players with memories restored have to re-write either their bond or their ideal with something in favour of the fates:
-- "The Fates have chosen me to be their herald"; "The Fates saved me when no one else could"; "Giving myself to the Fates is the best decision I ever made"; "The Fates love me like no one else ever will."

The players will insist this is who they always were.

 

[jayoungr]

Another option is the Total Recall scenario, where their missing memories reveal them as having been someone they don't want to be.

They lost their memories during the game, though, so there isn't really a chance to surprise the players in this case.