Sacrosanct
Legend
Changing the backstory of the world to account for the PCs runs perilously close to meta-gaming. It's better to just forget about these characters, in order to avoid overstating their importance.
Perhaps it would help if I gave an example.
A few years ago, I was rerunning a group through ToEE. One of the PCs backgrounds was that he worked in a circus, and he and an orc rival both wanted to be with this one person The orc was evil, violent, and overall not good news, but convinced everyone else he was not a problem. In a fit of jealously, the PC ended up getting into a fight with the orc and killing him (albeit accidentally). He was forced to pay for the Resurrection costs and be exiled from the circus.
In the temple, there's this bandit leader general. So I changed the bandit leader to be this half orc (who left the circus to fulfill his desires of crime and violence). The orc ended up running into the PCs a few times throughout the adventure but didn't reveal himself until later in a typical "big reveal" trope.
In another example:
The PC's background was that their mother will killed by a rival warlock cultist and are hunting him. The adventure main plot is around a bunch of cultists rising a lost god (very Lovecraftian feel). So it was very easy for me to make those cultists responsible for his mother's death and hunt for him, and change some of the NPCs around to fit his background more closely.