Here's a background on how he became what he is today, a Steathy Mageknight, if it'll help you with a decision
I'm almost guaranteeing I'll change it, but it will still revolve around a powerful evil entity forcing its way into Aranion's being.
Aranion is a good person though, and does not want for this evil inside him to be a part of who he is. He always believed in order, and that there was always a proper goodness that the world was meant to have. He wishes to rid himself of this evil, and continue on his normal old life on the path of good.
I've been trying to think of the deity he'd act for as a Paladin, and have figured out a few, with 3 being my particular favorites, in the order that I like them:
The Path of Light - Maybe Aranion was already a follower of The Path of Light, and actively sought to better himself in both mind and body; hence him being able to focus his ki (from the multiclass assassin), being so knowledgeable (scholar theme), and having a strong body (Paladin; able to actively use swords and use heavy armor). However, when he was tainted by the impure power that forced it's way into him, he was cast out of his place of teaching, and forced to be on his own. Now he travels the land, continuously following the teachings of The Path of Light in hopes to vanquish the evil that resides within him and become pure once again.
Nusemnee - Maybe Aranion was corrupted by the evil power at a very young age, back when Nusemnee was still alive. He was naturally hated for his ominous presence due to the evil inside him, and casted out of the village where he lived. Seeking redemption, he thought of Nusemnee with aspiration; if she could turn to the good, despite her evil background, why can't he? He will make good of himself, even if it costs him his life. He follows the teachings of Nusemnee in search for redemption, and to hopefully make a name for himself as a good person.
Ioun - Maybe Aranion was an incredibly gifted scholar, who studied basic arcane arts, swordplay, history, language, and other scholarly things. Yet maybe he looked into things that he shouldn't have, and exposed himself to an evil so great, that it consumed him and stayed permanently inside him. He already revered Ioun as the greatest deity, being that he valued knowledge, but knew nothing of how to get such an evil out of him. Knowledge is power, and he is too weak to distill the evil inside of him, so he quests for the knowledge in the name of Ioun, god of Knowledge, in order to find the answers he needs.
I particularly like The Path of Light one, because his conflict with his own 'religion' of The Path of Light reflects his own inner turmoil with his path of rightchousness. But I'm not entirely sure if that would be a sensible choice because, quite frankly, I'm new to the D&D lore I really wish I could sort of combine The Path of Light and Nusemnee ideas, but The Path of Light teaches that you shouldn't look to deities :/ But since the teachings only say "Do not look to deities for divinity.", maybe I can still use Nusemnee if I say he looks at her as a source of inspiration from her story, not as a form of divinity worthy of worship? The Path of Light would still be his following that he acts as a Paladin for, but he actively reveres Numsemnee and pays her tribute just for the sole fact that he respects her going through what he is going through. Does that make sense?
Other deities that may be good for consideration are Dol Arrah, The Silver Flame, Torm, The Sovereign Host, Ilmater, and Mielikki. Although, I don't think any of those would work as well as the above three.
What do you think sounds the coolest? Also, any background ideas are more than welcome!