WARNING: The following contains spoilers for Season 4, Episode 5 of Attack on Titan, "Declaration of War," now streaming on Crunchyroll, Funimation, Amazon Prime and Hulu.
One of Attack On Titan's biggest mysteries has finally been solved in the latest episode of Season 4. We've known since Season 1 that there were Titans in the walls surrounding the inhabitants of Paradis Island, but Season 4, Episode 5, "Declaration of War," clears up why they're there and what their presence really means both for the inhabitants of Marley and for the world at large. And, as is often the case in Attack on Titan, the reason is far from good.
The episode follows two main threads: one is the dramatic reunion between Eren and Reiner, and the other is Willy Tybur's performance and declaration of war above them. While Willy confesses the truth of the Eldians' real history to the citizens of Marley and other world diplomats, he divulges the truth of the Wall Titans, too. He explains that King Karl Fritz used the Founding Titan's powers to summon unnumbered versions of the Colossal Titan, using them to build the walls surrounding his people. These walls were aptly named after Ymir Fritz's daughters: Maria, Rose and Sheena. (Ymir being the 'mother' of all Titans.)
The Wall Titans have different abilities from the regular Colossal Titan. Each of them has the ability to not only harden the top layer of their skin but also almost the entirety of their bodies. Using this method, they lined up shoulder to shoulder and hardened, 100 years ago, creating the walls we know today.
We're first introduced to the Wall Titans as far back as Season 1, Episode 25, "Wall: Assault on Stohess, Part 3." While Annie tries to escape from Eren during the battle in the Stonehess District, Mikasa cuts her off while she's climbing the wall. Although Annie gets captured, she damages a big portion of the wall, revealing a horrifying face inside of it; one that's still quite alive as we can see its eyes follow Mikasa. The breach is quickly covered up as Minister Nick suggests that exposure to sunlight will wake it up. Beyond that, no further information about them was given in that episode.
The next time this phenomenon was mentioned was in Season 3, Episode 7, "Wish". While Rod Reiss tries to convince Historia why it's necessary to eat Eren as a Titan, he quickly explains how the power of the Founding Titan is what created the cavern they were in, as well as the three walls. But Rod doesn't ever mention the Wall Titans or how specifically the Founding Titan made it happen.
After that episode, we don't find out about the actual Wall Titans until the secrets in Grisha and Eren's basement are uncovered in Season 3, Episode 20 "That Day." But even then, it is not fully explained exactly how these unique Titans operate or what their purpose is.
In Willy Tybur's speech, he explains that when King Fritz built the walls he made a vow with the Founding Titan, renouncing all future war. But to the nations on the mainland, he issued a warning: tens of millions of Colossal Titans would be unleashed on them if ever an attack was made upon the Island. This was the main deterrence and the reason why Marley never openly attacked them. If they did, they could expect an apocalyptic event that would not only destroy them but potentially the world. Willy calls this horror, if unleashed, The Rumbling -- the stampede of millions of Colossal Titan footsteps.
Willy continues, calling naming Eren Yeager as the source of a recent uprising on Paradis Island. He tells his audience that Eren has the Founding Titan, which means he can unleash the Rumbling upon them all. Using this fear as the motivator, this is how Willy Tybur declares war on Paradis Island -- by declaring war on Eren Yeager.
Ultimately, these Wall Titans pose a serious threat to both Marley, the world, and even the inhabitants of Paradis Island. Will Eren truly unleash the Rumbling or will the path he's going down change course before the anime is finished? The stakes couldn't be higher in Attack on Titan's final season.