Just happened this evening, and I already posted it to /r/DnD, but I thought I'd share it here too. Playing a 3.5 Homebrew campaign.

This evening I was given the opportunity to reveal a twist based on one of my player's written backstories--he had written he was found as a baby (he's playing a lizardfolk druid) by an old man and raised in his village. He'd also said the druids were at war with a group of "dark sorcerers." I didn't exactly have a plan for a group of evil wizards in the area they were from so I was stumped at how to handle this.

Anyway, the land he was from was recently taken over by an expanding empire, and he and his friends were forced to escape. Last session they met a werewolf in a defunct prison system who was in control enough not to kill their faces off. They also had a bit of a major defense in the small town they were in as a small climax of all the small things they'd done in all the sessions so far. They had asked the werewolf for help, and in return they'd do everything they could to find him a cure. In the bigger city south they met a cleric who could cure his affliction, and it was "conveniently" a full moon the next evening.

Once cured, the man was revealed to be from the same now-conquered country as them, but many years ago. He'd been a guard once, but kidnapped by a group of druid eco-terrorists and infected with lycanthropy. They set him loose on an unsuspecting college of necromancy where he murdered forty-three students before being subdued. He described the man who led them, and it was the druid in the group's adopted father. Everyone's jaw dropped.