Rumbling (Anime)

"The threat of Paradis Island is the stampede of millions of Colossal Titans—a rumbling. As I mentioned, the vow renouncing war prevents the King's bloodline from fully using the Founding Titan. But currently, this "Eren Jaeger" who possesses the Founding Titan may begin this rumbling at any moment. Once the rumbling is set into motion, there is nothing that we can do. Nothing but tremble at the footsteps of doom, fleeing in vain. All lands and civilizations will be trampled, literally razed to the ground."

- Willy Tybur explains the destructive potential of the rumbling to his audience during the Liberio festival

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 is a key deterrence factor for the Subjects of Ymir on Paradis Island.

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 Sina. These Walls were constructed from the hardened flesh of countless Colossal 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, the threat of the rumbling was too great a risk to attempt a full scale invasion. Instead, Marley began training the Warriors 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 Eldia Restorationists led at this time by Grisha Jaeger attempted to use this Warrior program to infiltrate the Walls in the same fashion in order to steal the Founding Titan to restore Eldia.

Clash of the Titans arc
During the Scout Regiment's attempt to capture Annie Leonhart, a portion of Wall Sina 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 Scout Regiment and panic in Pastor Nick of the Order of the Walls, demanding the scouts to cover the face of the Titan before sunlight could reach it. The presence of the Titan in Wall Sina leads Section Commander Hange Zoë to quickly deduce that all of the Walls must contain Titans within them.

Return to Shiganshina arc
Upon discovering the journals of Grisha Jaeger in the Jaeger family's basement in Shiganshina District, the Scout Regiment 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 Jaeger 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.

Marley arc
After the conclusion of the Marley Mid-East War, Willy Tybur is recruited by Zeke Jaeger 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 Jaeger 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.