Unseen
The so-called 'affliction' of the Unseen comes from the old Udynean belief that only the firstborn in a multiple birth has a soul. Traditionally, the belief demands that any child born beyond the first in a single birthing is to be killed to appease the gods. The known few who survive this custom are considered to be invisible, thus Unseen.
Firstly a belief that was wholly human, its influence over the centuries has crept into the mindset of all Udyneans, be they human, elven or half both races.
History
This belief is old enough to have records from back before the forming of the empire, as well as a variant within the enemy kingdom of Obuzan. Whilst there are no clear reasons for it, many Udynean scholars suppose that it likely came about during the Nulling Plague that persisted from 301-286 HC when Nulled Ones were uncharacteristically abundant and childbirth became a risky act, with the birthing parent often surviving the delivery of the first child, but failing when it came to their twin.
Cultural Reception
It is theorised that a great deal of the children found on temple doorsteps are a result of parents seeking a way for their children to live without the stigma of being Unseen as they are considered to not exist. This means they cannot inherit land or other assets, officially collect income, become married under the government's eye or even be considered a parent of the very children they might birth.
Conversely, this also means they cannot be held accountable for any crimes they commit, from simple thievery to murder. Because of this, many known Unseen are recruited into the assassin guild if they do not find themselves victims of a city's gang, forced to work for them without any means of escape.
Since the current Mhanek refused to kill his daughter, and the vris Mhanek likewise wouldn't let either of his first two sons be sacrificed, there have been many rumblings amongst the people and talks of officially abolishing the tradition. Whether the motion is successful and how long it would take for Unseen to finally be treated as normal folk is yet to be determined.
I'll keep my fingers crossed that the motion is successful. It must be awful for parents to have to choose between killing their children, giving them away, or just be forced to pretend they don't exist. I liked the detail of how the treatment of the unseen ended up biting their society in the rear, simply through going so far and deep as to view the unseen as more or less equal to air. Though, really unfortunate for the affected individuals to be pressured into criminal careers due to this loophole. A really cool concept for a "cultural-contracted" condition.