The Father by
If you've ever wanted to read a story about Darth Vader being baffled and enraged (and eventually redeemed) by his young teenage offspring, have I got a story for you! 230k words of an AU where Luke and Leia (and Han and Lando) get tangled up in Vader's business about five years before actual canon, forcing Vader to confront his feelings about fatherhood, the Emperor, and his own sins. There's also a lot of action and some deeply hilarious scenes. It's on FF.net, so some caveats may apply, but I enjoyed this a lot, despite the length.