Many elements in Attack on Titan (AOT) are shrouded in mystery, one of them being Eren's connection to royalty. However, the recent seasons of AOT have peeled off the layers of mystery and revealed Eren's linkages to royal blood, which allows the full use of the powers of the Titans.
Considering how important the element of Eren's connection to the royal bloodline directly descended from Ymir and his role in the series finale is, his role and status are something that play a pivotal role in the story's conclusion because of the Founding Titan and its ability to control Titans and subjects of Ymir.
Due to the use and implementation of the Founding Titan in the Attack on Titan series, Eren's connection to royalty is pivotal to understanding the plot. Although not a direct descendant of King Fritz and a former Eldian soldier, Eren is connected to the royal bloodline in a unique way.
Disclaimer: This article contains spoilers for the Attack on Titan series.
Unraveling Eren Yeager's connection to royalty in Attack on Titan
Eren first activated the power of the Founding Titan through his Coordinate Ability after coming into contact with the Titan form of Dina Fritz, Zeke Yeager's mother and the last surviving member of the royal family in Liberio. This connection offered viewers a glimpse into the connection between royal blood and the Founding Titan.
The Founding Titan is one of the cornerstones of the Attack on Titan worldbuilding. Ymir Fritz, the founder of the Titans, was connected to the royal blood through her marriage to King Fritz. As the founder, she is able to connect all the subjects that descended from her through the paths that converge at one place, a physical world that defies the rules of real-time space and time.
Thus, although Eren does not have a real royal connection to the descendants of King Fritz, through his half-brother Zeke, he is able to control the Founding Titan and activate the Rumbling. Thus, once the latter was killed, the aforementioned Rumbling stopped.
Eren Yeager's legacy as an anime protagonist
Eren Yeager's role in the Attack on Titan series is quite unique in the anime community because he goes from a fairly standard protagonist to the story's main villain. While viewers can understand his motivations and where he is coming from regarding The Rumbling, it's amazing how Isayama made him go from a somewhat idealistic lead to a full-fledged nihilist.
It is a testament to Isayama's writing skills that Eren progressed from simply wishing for the ability to roam outside his city's borders to actively fighting the war.
Final thoughts
Attack on Titan has a lot of mysteries and questions, and Eren having royal blood could lead to some confusion because of his connection with Zeke, but that is not what a lot of people think. He didn't have any royal blood and had to rely on his half-brother to be able to use the Founding Titan.