Alright, so this campaign takes place in Lamordia. one of my player's backstory villains is a priestess of osybus that basically raised the pc to be an acolyte/mindless killing machine from birth. I'm having a bit of trouble coming up with this character's goals and personality though.
The basics are, the pc is a lizardfolk ranger that was trained in the shadowy tattoo magic that the priests often used. Oftentimes she was buried underground among corpses or left behind in dangerous locations (sleeping beast mountains, bogs, etc) to make sure she could survive these places and to strengthen her connection to death.
Eventually, she tasked the pc with killing a party of adventurers (not the pc's of the campaign) who happened to wander into the mist, but the party defeated and befriended the pc instead. The pc hasn't gone back to the priests since. The backstory party still ended up dying, but much later to a powerful monster, and the pc is actually traveling with the party the campaign is about now.
Also for reference, I did have a priest of osybus as a villain early on in the campaign. He had broken off from the main sect though, because he was convinced that he could "create" the perfect vessel for the dark powers to inhabit, instead of corrupting someone sufficiently like their task normally is. Which I felt was very lamordian.
But, yeah, I guess I'm drawing blanks as to this lady's motivations and how she might hinder the party (who's goal is to find van richten). Obviously she'll want her 'weapon' back. But besides that, I don't know how she can be unique from other priests in terms of her goals and backstory/personality.