I have some backstory which has come out of my campaign and which I'd like to share. It's not complete, but I think even in this unfinished state it might be able to inspire others to create and share some of their backstories.
In my campaign we have two characters whose history ties into Roslar's Coffer. The bard, Deci, was a refugee from the orc raid in her childhood and grew up away from the town. The party cleric, Nico, is a cleric of Lastwall. His father, Regio, was a paladin of Lastwall and killed in another raid on Roslar's Coffer after the big one. (He was there because of increased patrolling following the razing of the town in Deci's raid).
Regio was interred in the town cemetery. A mid-level paladin, his skeleton was also turned into a graveknight as a result of the Radiant Fire (there are reasons for this which I will get into later). There was a 'family reunion' of sorts at the bitter end of the tomb section of Book 2 wherein Regio tried to tempt Nico into embracing undeath and finally be the son his father always wanted (there's a Boromir - Faramir vibe going on here). The conversation hinged on the father's disappointment of the son (for not following in his footsteps) and the son's resentment of the father (for paying more attention to duty than family). During the conversation, I was subconsciously seeking a way to make real this debate through some background experience. It was in this light that Regio answered one of Nico's rebukes with the phrase. "Yes, I always obeyed orders.. just like at Kassen." In hindsight, I think this was a wonderful thing to say.
I told my player that Nico remembered "Kassen" as a word that was thrown around during a dark time in the family history over a decade ago - when Nico was only seven or eight. Kassen, Nico would know, was a small town east of Roslar's Coffer (see the Crypt of the Everflame module). I explained that Regio came home, uncharacteristically took to the bottle over several weeks as officials, police and officers had several heated discussions. As an eight year old, he didn't remember much, but he heard the name Kassen used frequently.
The backstory I'm going with is that Regio was on an undercover mission - a deep undercover mission - to infiltrate an important Whispering Way cell in Nirmithas near Kassen. To be convincing as a fallen paladin looking to join he had to actually fall. And that meant slaughtering his guard outpost while they slept. It also meant preventing word about the impending raid at Roslar's Coffer getting to the right people allowing the big raid to occur (and turn Deci into a refugee - its just a happy coincidence that this all occurred in the same year). Now, none of these deeds were ever expressly ordered by the high command - Regio was told to infiltrate this cell regardless of what it took. The cell contained a high level Whispering Tyrant person - someone along the level of Gildeas (though not Gildeas himself). The sting was successful and Lastwall was saved, but there was private horror at the cost, even though at the highest levels, there was approval and Regio was never charged with any crimes. He was, however, given a desk job in Vigil for the next several years and it took a long time to regain his paladin powers.
Why is this so important? Because Regio has a part to play in this AP as well. It's a reprise (and here we get to that 'later' part I alluded to earlier). Because Regio's shown a knack for doing unsavory things in the name of good he was the perfect person for Iomedae to send back and allow to be transformed into a graveknight. Suitably clad, he can work from within to help bring down the WT. After all, just because they are evil doesn't mean they are united. And just because they are good doesn't mean they always make "best choices". Ask Arazni.
So, a few off hand remarks, some thought after the fact and a good sense of the spatial and temporal relationships of the AP made this a fun side story to piece together. This is one of the things I like doing most when GMming - putting together these supporting characters. Now i have the opportunity to further explore this through Nico's family, Lastwall officials, etc... And I have a great recurring character whose motives are morally ambiguous. (the party has most, but not all of the graveknight's armor.. there'll be a return of our villain).
So who else has some fun backstory for their campaign?