Backstory for a Bladebound Magus' Black Blade

[Timeadol]

Hello!

I would like to flesh out the backstory of one of my player's Black Blade and hope that you can help me with it as I'm not very familiar with Golarion and its history.

The player decided to change her Magus to the Bladebound Archetype just before their group started exploring Harrowstone. At this point she already owned a masterwork longsword she wanted to turn into her Black Blade and we agreed that I would come up with a backstory and mission that would fit into the Carrion Crown adventure path. The weapon had been bequeathed to her in Petros Lorrimor's last will but didn't have its own story then.

This is what the Magus knows so far:
In the basement of Harrowstone the blade activated when soot covering the walls suddenly rose up and bonded with the sword turning it black.
The sword claimed to have no memory of its past but soon remembered its name to be “Caz-Duhul” although it didn't know what it meant.
The Magus questioned Ravengro's smith Jorfa about the sword who told her that it was of dwarven craftsmanship although she didn't know where and by whom it was forged. However, she was sure that it was more than 200 years old and that the name “Caz-Duhul” was not dwarven but probably ancient Varisian.

These are other “facts” I would like to use for the Black Blade's backstory:
it was forged ca. 3.800 AR by the dwarves of Kragoddan
a dying dwarf gave it to a Knight of Ozem during the Shining Crusade
it became somehow a special weapon when its owner fought alongside the demigoddess Arazni after she was summoned by the Knights
it's purpose is to fight the Whispering Tyrant and the Whispering Way
it is inhabited and powered by a soul that also gives advise to its wielder and that can be released when it has completed its mission
the current soul inside it belongs to Petros Lorrimor who will regain his memories over time

The group is now in Lepidstadt for the Trial of the Beast and I intend to give the Magus a little bit more information via Dr. Montagnie Crowl and maybe Kendra Lorrimor who is reading her dead father's records.
Dr. Crowl can roughly translate the name to “spirit trap“ or “soul vessel” while Kendra will find out how Lorrimor acquired the sword.

Does anyone have a suggestion how the sword ended up in Lorrimor's possession? A painting at Schloss Caromarc or a member of the Esoteric Order of the Palatine Eye could provide some information but what exactly? Maybe there were other owners that wielded the Black Blade? Are there any NPCs or organizations in later volumes that can provide clues to its past?

Thank you for your help!

[Sibelius Eos Owm]

Meant to respond to this a lot sooner than I ended up but it does look like you're mostly on top of the history here. I don't have a specific answer for how the sword might have ended up in Lorrimor's possession but the dude is fantastically well-traveled as suggested by the campaign traits, so he can have picked it up in his studies virtually anywhere. In particular Lorrimor is a Pharasmin with an interest in history, magic, and in studying the forces of evil to learn how to defeat them. The moment he learns this sword has unique properties and was wielded in the Shining Crusade it becomes something he might be interested in.

It is highly plausible there have been other wielders in the last 3500 years depending on where the sword got lost (see Raven's Head for an artefact from the Crusade which was lost at the bottom of the sea for this duration). As for major NPC organisations, the Esoteric Order are pretty much the best bet. They return as a minor but significant part of Book 5 (and if you're like me Kendra is in Caliphas, too, helping research). Perhaps a member of the Whispering Way (Auren Vrood himself?) might have some kind of knowledge from the end of Book 3 if looking for something much sooner.