If you are a player in "The End of Days" then stop reading
So, the long and short of it is that my pcs are traveling to the midnight isles to retrieve a book once owned by an angelic ally of theirs and leant to one of his contacts. They do not yet know that this book was the Concordance of Rivals, this angelic ally is in fact Tabris, and his contact in the Midnight Isles was Nocticula herself.
So, nocticula transformed into the redeemer queen only a few years ago, as such neither the pcs (who've been hibernating) nor Tabris (in hiding) are aware of yet. What would have been a tense but quick exchange of studying materials will now be a search.
Luckily Tabris formed a pact with Nocticula, such that she could never take the book from the midnight isles or damage it. As such they know it's in the isles.
Eventually they will learn that two former members of Nocticula's courts are fighting over it.
The first is a Lilitu 'Priestess' of Nocticula-that-was who is now trying to establish a cult formed around herself. She believes that information in the Concordance was necessary for Nocticula's ascension to godhood as a neutral deity. She wants the book so she can find how she did it, and eventually claim the power Nocticula shed as she ascended and take over her former sphere.
The other is a Succubus wizard of some skill. I'm thinking a necromancer. I am uncertain what her motivation should be though. It should be of similar scale to the other. I originally thought she would use it to force an usher to judge one of her mortal victims so that he'd go to the abyss so she could finish the job, but that seems pretty small compared to the Lilitu.
TLDR I need a motivation for why a powerful Succubus would want the concordance of rivals that is of similar scale but distinct from attaining demi-godhood.
Revenge. On a diabolical scale. Something that involves intricate knowledge of the Boneyard to pull off.
Intercepting/stealing/stopping an important soul from crossing over?
I don't see any Succubus getting an upper hand on Pharasma, nor do I see a reason for her to even try. But stealing a soul if it really screws over someone she hates might be worth it.
The soul in question could be a bargaining chip for her to blackmail some powerful Demon, or whatever.
Or maybe it's all a grand deception, and she is figuring out how to trick psychopomps into doing her bidding. But, once again, as soon as Pharasma figures it out, the Succubus is toast.
Playing power games with Demons is what Succubus do... going toe to toe with possibly the most powerful deity, whether you have a fancy book or not, is not how you go about seeing your retirement.
Hmm... I suppose that makes a good deal of sense. Just an expansion of what had been an earlier idea.
An alternative idea is perhaps she could be searching for a way to gain power over the method of petitioners to the Abyss. Maybe she could find a way to make choice abyssal petitioners remain humanoid and lucid, perhaps even retain their memories, but also under her control. She can then make them into even more powerful demons, as their powers are concentrated into one entity.