WARNING: The following contains spoilers for Season 4, Episode 10 of Attack on Titan, "A Sound Argument," now streaming on Crunchyroll, FUNimation, Amazon Prime and Hulu.
Attack on Titan's plot has undergone more than a few wild plot twists since its early days. In its inception, this series was a sort of mecha/zombie horror cross, where humanity desperately defended its last walled city from the Titans. But all that was a lie, and the true origin of the Titans is becoming clear. All the lies are falling apart, partly thanks to Historia Reiss.
Historia, introduced as Krista Lenz, is no ordinary Scout. She is a member of the Reiss royal family, which is connected to the Fritz royal family. Thus, Historia is distantly related to Zeke Yeager, and is revealed in Episode 10 of Season 4 to be a major part of Zeke's current plan. But it's a rather quiet, not to mention bleak, role -- a far cry from her former moment of glory.
What Historia Reiss Must Do in Attack on Titan's Final Season
Paradise Island's time is limited. Before long, the mighty Marley Empire will invade to seize its resources and wage war on its hated Eldian "devil" inhabitants, so it seems as though it's up to the likes of Zeke and Eren to muster the island's Titan defenses and discourage all attempts at invasion. As the brothers argue, the Scouts alone won't cut it; instead, the Eldians need to perform the Rumbling -- which involves using the walls' countless colossal Titans as a nuclear deterrent. How can those Titans be controlled? With royal blood and the Founding Titan, which each Yeager brother has. But there is a snag, one that's built into Historia's royal genes.
King Fritz ensured that a royal wielder of the Founding Titan will desire peace and stability, and never use the Titans for war. In fact, the royal bloodline has kept the Eldians ignorant of their true history, and Zeke knows all this. So, the task of performing the Rumbling has been forcibly split between two parties, with a royal Titan and the Founding Titan being two different people. This allows the Founding Titan's power to be used without the built-in influence of royal blood. Currently, Eren Yeager currently has the Founding Titan (as well as the Attack and War Hammer Titan), and the Beast Titan has now been decreed by Zeke to be passed from him to Historia, from one royal blood bearer to another.
Of course, a Titan Shifter can only live for 13 years once he or she obtains the Titan's power, and Historia must keep the peace for a longer period than that. So, it will fall to her descendants, and that means having those descendants. Historia's greatest value now is not her leadership or combat abilities, but her genes and body.
Historia Agrees To The Hiruzu Deal
As of now, Historia Reiss is the queen of Paradise Island, having foiled her father Rod's plans, and she is more assertive and sure of herself than ever (this is partly thanks to Ymir's influence). However, Historia's position is largely symbolic, and now, it might mean almost nothing at all. In "A Sound Argument," delegates from the nation of Hizuru arrive as friends of the Eldians. An ambassador Azumabito (Hizaru's most prestigious family) desires the ODM's Iceburst Stone source and promises to help Paradise Island modernize its defenses, but modern armies alone can't defend the island. Historia must get ready to have multiple children right away, according to Zeke, so the Reiss bloodline can be housed in multiple people, and outlive Historia herself.
Eren wasn't thrilled about this plan initially, but with Titan serums on hand, the plan is feasible, and Historia agreed to it at once. She is now intended to obtain Zeke's Beast Titan, and as a royal Titan, she can help Eren use the Founding Titan for purposes of war and circumvent King Fritz's failsafe. Alone, Historia can only make this possible for 13 years, so her Beast Titan must be passed on to a child of hers, and that child will keep up this duty for 13 more years before passing it on to a brother or sister. The more children Historia has, the better -- until those children are old enough to have their own children and continue the Reiss/Fritz bloodline further.
All this means that Historia, the girl who freed herself from her family's shackles and became royalty, is now a bloodline propagator... and not much else (unless the plan changes). In fact, she isn't even physically on the throne anymore. Instead, she's revealed in the latest episode to have retreated to the countryside in seclusion, and as of now, she is pregnant with an unnamed man's child (likely the first of many pregnancies on her part).
Historia's genes are vital, but it seems that the woman herself is not. Historia rose to the top, only to fall back to the bottom, and Eren and his friends can only guess how Historia feels about that. Is she bitterly disappointed? Proud to play such a vital role? Confused? It's not yet clear. But it is a real shame.