There's a Rumbling in Attack on Titan season 4, and it can't be Sasha's stomach anymore. Currently in Attack on Titan, Eren Yaeger and Survey Corps have recently returned from a devastating assault upon the Marleyan mainland. By sneaking behind enemy lines and secretly inviting his allies to the party, Eren took out Marley's military leadership, stole their War Hammer Titan, and "captured" Zeke, the owner of the Beast Titan. Although this might appear like a landmark victory, there's clearly some disagreement among Eren's friends as to whether a direct battle was the wisest course of action.
Everyone seems to have their own agenda in Attack on Titan season 4. Paradis Island welcomed Zeke's Marleyan rebels and the two sides formed a strategy together, but Eren and Zeke have been conspiring in secret and deviated from the script with the recent Marley attack. Meanwhile, Zeke's followers were placed under arrest upon their return, suggesting the leaders of Paradis have intentions of their own. Despite so many conflicting plots, all of them revolve around something known as "the Rumbling." Some people want it, others want to prevent it, some simply wish to exploit the mass fear this phrase instills. With Attack on Titan's final season moving faster than Hajime Isayama's original manga, however, the Rumbling hasn't been explained as fully.
The first inkling of the Rumbling came in the final moments of Attack on Titan season 1, when a Titan's face was shown inside Wall Sina. Since then, Attack on Titan has revealed that all three giant circular walls on Paradis Island are constructed from millions of massive, hardened Titans. Although these Wall Titans are similar in size and appearance to the Colossal Titan, the creatures are quite different from the unique Colossal Titan possessed by the likes of Bertholdt and Armin. Thanks to the history lessons provided by Grisha Yaeger and Willy Tyber, it's known that King Fritz built these walls using the power of the Founding Titan after evacuating to Paradis Island following a long, bloody war with Marley.
Fritz biologically bound his people to an oath of peace, but publicly threatened that if Paradis was ever attacked, he would unleash the millions of Wall Titans and have them trample across the globe. This is what's widely referred to as "the Rumbling," based on the sounds and shaking these massive giants would cause as they walked the Earth. With so many big titans rampaging ceaselessly, the Rumbling is undoubtedly an apocalyptic event, but three conditions are required. Firstly, the power of the Founding Titan, currently residing within Eren Yaeger. Secondly, if the Founder is not of royal descent, another Titan possessing King Fritz's bloodline is required. Without royal blood, Eren cannot use the Founder's full power. Finally, even though the Rumbling needs royal DNA, a member of the Fritz family can't trigger it themselves due to Fritz's renouncement of violence.
This is why Eren and Zeke being together is so important. For the first time, the Survey Corps is in the driving seat. Marley now knows Paradis Island has the means to trigger the Rumbling, with Eren holding the Founder (but not being of royal blood) and Zeke's heritage allowing Eren to access the Founder's power. An alternative would've been to transform Historia into a Titan and use her in Zeke's place, but it's unlikely Eren would agree to this due to his friendship with the Queen.
Of course, the Survey Corps isn't actually planning on ending the world, so what was the point in sacrificing so much to bring Eren and Zeke together onto the same island? Right now, the official plan is to use the Rumbling as a threat. Marley should be too scared to attack in case Paradis activate the Wall Titans, and this gives the islanders time to build their forces, make allies, and ready themselves for war. But Zeke's presence on Paradis creates two complications. Will Historia really be left out of the Rumbling plan, as per Eren Yaeger's wishes? And might there be at least one person in Attack on Titan who sees the Rumbling not as a mere threat, but as a necessary course of action?