The Rumbling was a cataclysmic event involving Karl Fritz's Wall Titans marching across the earth, destroying all life upon it. For over a century, it was a key deterrence factor for the Subjects of Ymir on Paradis Island until the year 854 when Eren Jaeger unhardened the Walls, freed the Wall Titans, and began the Rumbling.
Over the course of four days after first setting foot on the continental mainland, the Rumbling brought about the complete destruction of the northern continent of Marley along with significant devastation in the southern continent and neighboring nations. Approximately 80% of all human life on the planet was ended by the time the Rumbling was stopped prematurely by the combined efforts of the remnants of Paradis Island's Scout Regiment and Marley's Warrior Unit.
Prelude: 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, each named for the three daughters of Ymir Fritz. 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.
Karl Fritz's ultimatum: 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, each named for the three daughters of Ymir Fritz. 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.
Truths rediscovered: During the Scout Regiment's attempt to capture Annie Leonhart, a portion of Wall Sina in Stohess District was damaged by Annie's Female Titan, revealing the face of a Wall Titan within it. This revelation caused 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 led Squad Leader Hange Zoë to quickly deduce that all of the Walls must contain Titans within them.
Memories of the future: During a ceremony honoring the survivors of the Battle of Shiganshina District, a moment of contact between Eren and Historia opened a glimpse into Eren's future memories where he saw 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 saw the sight of the Rumbling that he would cause in the future and his inevitable defeat after annihilating eighty percent of all human life. One year after the medal ceremony, Paradis Island was visited by the Anti-Marleyan Volunteers, a secret organization of Marleyan soldiers led by Yelena, all foreign conscripts who wished to aid Eldia against Marley with the help of Zeke Jaeger.
Source: Rumbling (Anime), available under CC BY-SA 3.0.