Board Thread:General Discussion/@comment-27565137-20160514033814/@comment-27392440-20160515133850

Though I can't give a factual answer (not enough information in series) these are my thoughts to the answers:

Yhwach likely created the quincies to create an army to conquer things, based on his goals and character.

Early in the series Rukia implied all hollows may have an acidic effect in their touch/presence like Orihime's brother did, so considering quincy pull external spirit energy into their bodies it's possible they're also pulling in that acidic presence a hollow has and being eaten away by that acidic effect entering their system over time.

Unless I'm mistaken it's been said or at least implied that all quincy are descendents of Yhwach by blood, so the first quincy being made/born may have been an unexpected event even for Yhwach.

Though it's been shown he chooses the letter there is some evidence the actual power comes from the quincy's own nature. In As Nodt's case he was ruled by fear before he got his schrift yet it fit him perfectly and the other stern ritter have personalities and styles that match their schrift, so unless a schrift changes who you are as a person to match the power it makes sense the power comes from them (I even think a quincy should probably be able to develop a schrift without Yhwach as he as a quincy gave himself one with no help so another quincy should be able to do the same in theory...just harder with not being the soul king's kid)