Board Thread:General Discussion/@comment-6746577-20140619004320/@comment-6007020-20140619010420

Considering that the Quincy reduce a soul to non-existence (or, well at least reduce them to their base reishi particles), I doubt they have ever had a positive relationship. A much better assumption for their pasts is that the Quincy were spiritually aware humans who took maintaining the balace upon themselves, choosing to act on their own rather than just sit and obey the Shinigami who told them that what is going on is beyond their comprehension.

Years of misguided teachings gave all Quincy a warped view of the balance, seeing Shinigami as monstrous as the Hollows they slay. Yhwach himself advocated the idea, though he didn't really care for the idea behind the war as much as the thought of war itself.

But after the fighting began, his intelligence agents told him of the Soul King, who keeps the entire Soul Society running, and was indeed something beyond both Human and Shinigami. Yhwach perhaps thinks that assimilating such a powerful being would sate his hunger and permanately prevent him from becoming the blind, helpless thing that he once was.

Of course, that's just my opinion. I mean, considering for a short while Yhwach's forces had a being who could make you forget he existed, it wouldn't be much to think they sent him in to read files that no one should have access to, gaining information on the Soul Society's secrets.