Board Thread:Questions and Answers/@comment-45683175-20200504080825/@comment-38964308-20200506232334

Ymir's daughters (Maria-Rose-Shina) probably have been married to generals and nobility in the royal bloodline,that's why the power of the nine-titans is reserved to the "elite" of the empire,and in order to create an army for war purpose,experiments and genetical engineering were made to reproduce a replica of the Titans powers with success sometines (the power of the Ackermans) to "failures" (mindless titans),and after Eldia became a hyperpower,civil war between aristocrats for the throne erupted (the great titan war) which ended to weaken the empire,and then King Fritz gave all the secrets,experiments data to Marley except the Attack Titan which his holder somehow escaped and merged with "normal Eldians" in Liberio until he gave his power to Kruger.

The subjects of ymir are essentialy the relatives (the tribe of ymir),slaves in old era were abducted or sold in the market so it's easy for royal informants to localise from when Ymir belongs,they were probably forced or seduced to enter in Eldia's service(as slaves,experiments subjects,soldiers,even probably given noble status),they may even given royal privileges as the Kiyoma stated that being Subject of Ymir was a mark of belonging to noble status in Eldia's "Golden Age".