Rumbling

"If you ever try to interfere in our affairs, the tens of millions of Titans that sleep inside the Walls will surely flatten the entire earth."

- Karl Fritz's final threat to the outside world, recounted by Grisha Yeager

The rumbling (地鳴らし) is a hypothetical cataclysmic event involving Karl Fritz's Wall Titans marching across the earth, destroying all life upon it. It was a key deterrence factor for the Subjects of Ymir on Paradis Island until the year 854 when Eren Yeager unhardened the Walls and began the rumbling.

History
During the final days of the Great Titan War between Marley and the Eldian Empire, King Karl Fritz used the power of the Founding Titan to raise three Walls on Paradis Island: Maria, Rose, and Sheena, each named for the three daughters of Ymir Fritz. These Walls were constructed from the hardened flesh of countless Colossus Titans created with the Founding Titan's power.

Upon gathering the remnants of the Eldian Empire into the Walls and closing the gates to the outside world, Karl Fritz issued one final declaration which assured the enemies of Eldia that if Paradis were to come under attack, the Founding Titan would unleash the Wall Titans to trample the earth into ruin. However, Karl Fritz secretly confided with the Tybur family and the remaining Fritz family on the mainland that this ultimatum was an empty threat of deterrence. In truth, Karl Fritz made a "vow renouncing war" with the Founding Titan ensuring that none of his successors would use the Founding Titan to threaten the world again, allowing for Eldia to live in a brief paradise before their enemies would arrive to destroy Eldia as Karl Fritz believed they deserved to be. As Karl Fritz had hoped, no open declaration of war or act of aggression was made against the people of the Walls for over a century, though Marley would use the lands beyond the Walls on Paradis Island as a penal colony for criminal or undesirable Subjects of Ymir transformed into Pure Titans. However, over eighty years after the end of the Great Titan War, Marley's declining military industry turned the nation's attention towards Paradis and the fossil fuels hidden beneath Eldian lands. Despite the vow renouncing war being known to Marley through the Tybur family, the threat of the rumbling was too great a risk to attempt a full scale invasion. Instead, Marley began training the Warrior Unit in preparation for the Paradis Island Operation, an infiltration of the Walls with the power of the Titans that would not provoke the attention of the King of the Walls with military force. Likewise, the Eldian Restorationists led at this time by Grisha Yeager attempted to use this Warrior program to infiltrate the Walls in the same fashion in order to steal the Founding Titan to restore Eldia.

In the year 845, the power of the Attack Titan causes Grisha Yeager to inherit future memories from his son Eren, revealing the Fall of Wall Maria later that day. Grisha flees to the chapel of the Reiss family where he is influenced by the will of Eren in the distant future to steal the power of the Founding Titan and slaughter the Reiss family. The future memories of the Attack Titan reveal to Grisha the sight of the rumbling that Eren will create.

The Female Titan arc
During the Survey Corps's attempt to capture Annie Leonhart, a portion of Wall Sheena in Stohess District is damaged by Annie's Female Titan, revealing the face of a Wall Titan within it. This revelation causes great confusion among the Survey Corps and panic in Minister Nick of the Church of the Walls, demanding the Corps to cover the face of the Titan before sunlight could reach it. The presence of the Titan in Wall Sheena leads Squad Leader Hange Zoë to quickly deduce that all of the Walls must contain Titans within them. This creates unease among soldiers such as Armin Arlert, who wonders at the possibility of all Titans within the Walls emerging to walk.

Return to Shiganshina arc
Upon discovering the journals of Grisha Yeager in the Yeager family's basement in Shiganshina District, the Survey Corps learns of the nature of the rumbling as a deterrence factor that has kept enemies of Eldia away from Paradis Island for over a century until five years ago. During a gathering to discuss the findings of Grisha's journals, the leaders of the military in the Walls discuss how the power of the Founding Titan to release the Wall Titans may be achieved despite the vow renouncing war. Recalling his encounter with a Titan whom he later realized was Dina Fritz, Eren Yeager realizes he can still bring out the Founding Titan's power despite his non-royal blood as long as he is in contact with a royal-blooded Titan, though he keeps this realization to himself for the sake of Queen Historia Reiss.

Shortly afterward during a ceremony honoring the survivors of the Battle of Shiganshina District, a moment of contact between Eren and Historia opens a glimpse into Eren's future memories where he sees himself influencing the decision of his father Grisha in stealing the power of the Founding Titan and slaughtering the Reiss family. At this time, Eren sees the sight of the rumbling that he will cause in the future.

Marley arc
When faced with the possibility that he will have to infiltrate Paradis Island a second time, Reiner Braun thinks back to his time spent spying on the Eldians within the Walls before his entry into 104th Training Corps. At this time, Reiner reminded himself and his fellow Warriors Bertolt Hoover and Annie Leonhart that the threat of the rumbling and the fate of all humanity depended upon the success of their mission. After the conclusion of the Marley Mid-East War, Willy Tybur is recruited by Zeke Yeager to announce to the world the threat that Paradis Island poses in order to remove global hostility towards the Subjects of Ymir as a whole. During his speech before the international ambassadors and journalists, Willy reminds the audience of the many Wall Titans present on the island and the possibility of Eren Yeager unleashing these Titans upon the world. In order to prevent this apocalypse from occurring, Willy Tybur urges the leaders of the world to unite against Paradis in open war. In the aftermath of the raid on Liberio which soon followed Willy Tybur's declaration, Armin Arlert recalls Paradis's first encounter the Anti-Marleyan Volunteers, a secret organization of Marleyan soldiers conscripted from foreign nations who all wish to aid Eldia against Marley with the help of Zeke Yeager. Through the Volunteers, Zeke proposed cooperation with Paradis to bring Zeke into contact with Eren Yeager to begin a small-scale rumbling which will further deter enemy attacks until Paradis can reach a point of technological advancement sufficient enough to no longer rely on the power of the Titans for protection. While the various military leaders of Paradis were openly against cooperation with Zeke, a former enemy, Eren revealed his discovery of a royal-blooded Titan acting as a potential loophole for the Founding Titan's power, thus making the rumbling a possibility for Paradis to use in self defense.

Unnamed final arc
While Eren is imprisoned for his insubordinate actions taken to start the raid on Liberio, Commander Hange discusses their past introduction to Kiyomi Azumabito, ambassador from Hizuru and ally of Zeke. When discussing the requirements to make Eldia a self-sustaining nation on Paradis Island, Kiyomi had proposed the use of the rumbling as deterrence for at least a period of 50 years while Paradis caught up technologically with contemporary nations. Despite his foresight of the rumbling which would occur two years later, Eren refused to comply with Zeke's plan to use the rumbling and pass his Beast Titan to Queen Historia to ensure the perpetuity of the process, deciding instead to pursue diplomatic solutions for Paradis. Regardless, the rumbling would become a key factor in the military alliance between Eldia and Hizuru. One year later, Hizuru's trade proposals for Paradis Island failed, once again leaving them with only the rumbling as deterrence. As one final alternative, the Survey Corps embarked on an undercover survey to Marley in order to gauge global sentiments towards Eldia for themselves. Despite public favor increasing for Subjects of Ymir worldwide, Paradis Island remained a point of high hostility at this time, causing Eren to abandon the Survey Corps in order to pursue his own agenda.

Shortly after Eren escapes imprisonment, Marley launches an assault upon Shiganshina District in revenge for Liberio in order to take the Founding Titan from Eren. When Zeke arrives upon the battlefield, Marleyan forces desperately try to prevent Zeke and Eren from coming into contact and unleashing the Wall Titans. After learning of Zeke's true plan to use the Founding Titan to sterilize the Subjects of Ymir, Armin speculates on Eren's true motivation for conspiring with Zeke. Deducing that Eren would instead use the Founding Titan to unleash the Titans in Shiganshina's Walls, Armin trusts that Eren is on their side. When Eren and Zeke come into contact, an instantaneous moment in time occurs where Eren comes to understand that Zeke's will as a descendant of the royal family supersedes his own will as the Founder's carrier. Eren learns that the Founding Titan's power is dependent on the will of Ymir Fritz, forever enslaved to obey the will of the royal family who enslaved her in life 2,000 years ago. When Ymir prepares to carry out Zeke's will to sterilize the Subjects of Ymir, Eren urges her to reject her duties and act in her own will.

Breaking free from her enslavement to the royal family, Ymir instead sides with Eren's will, unhardening all the Walls on Paradis Island. With the full power of the Founding Titan, Eren telepathically declares his will to all Subjects of Ymir that he has unleashed the Wall Titans upon the world to crush all life beyond Paradis Island.