WARNING: The following contains spoilers for Season 4 of Attack on Titan, now streaming on Crunchyroll, Funimation, Hulu and Amazon Prime.
In the Attack on Titan universe, the ability to become a Titan is meant to be the ultimate weapon. Such powers, handed down from the Founding Titan Ymir Fritz, allowed the Eldian Empire to rule for centuries. But technology is bridging that gap in the modern era, and the nine Titan Shifters need to watch their backs. Weapons such as the thunder spears are a serious threat.
For decades, the military of Paradis Island used ODM gear and paring blades to outmaneuver pure Titans and slash the napes of their necks, destroying the Eldians inside those Titans. But what about Titan Shifters such as Reiner Braun the warrior, or the War Hammer Titan, Lara Tybur? It's time for a new addition to the armory.
Developing The Thunder Spears
The concept of thunder spears was not public knowledge. These weapons were engineered by the military police corps, a secretive and elite organization. The survey corps and wall guards had no knowledge of the technology, let alone access to it. All this changed after the coup was launched and Paradis Island was reunited under Queen Historia Reiss, and the scouts got their hands on the thunder spears at last. Hange Zoe was particularly fascinated and explained these weapons to her allies.
Thunder spears are like steampunk missiles, being pointed metal tubes that contain high-explosive ordinance inside and rocket propulsion systems. As Hange demonstrated, a thunder spear is fired when the user pulls a trigger, and it self-propels toward the target with enough force to embed itself in a Titan's body. Even the Armored Titan's defenses can be breached this way, and then the thunder spear's second stage is used. Once embedded in the target, the spear is ready to explode. This happens when the user yanks on its cord and activates the inner mechanism. The explosion will drive the spear even deeper into the Titan, and the sheer force can blow the Titan apart.
Even Titan Shifters are vulnerable to this, and the hope was that thunder spears could drive a Shifter user out of their Titan body, making them easier to capture. It should be noted, though, that the user must get to cover before the thunder spear explodes, or risk suffering serious injury. These are not weapons to be recklessly used.
The Thunder Spears In Action
It didn't take long for the thunder spears to be field-tested against Reiner Braun and other Titans. A great battle for Shiganshina took place, and once again Reiner's Armored Titan was the #1 asset on the Titan side, so the scouts got to work. They embedded thunder spears in the Armored Titan's eyes to blind Reiner, then struck the him with a few more spears, punching deep into the Titan's head. Once the cords were yanked off, Reiner was blasted right out of the Armored Titan and rendered practically helpless. Despite some mishaps along the way, this first application of thunder spears easily proved the effectiveness of these weapons.
The thunder spears saw action once again four years later, when Eren recklessly assaulted the Marley city of Liberio. This time, the scouts used them against all their enemies, human and Titan alike, and the thunder spears blew away Marley's anti-Titan emplacements to protect Eren from their wrath. The thunder spears were also used against Lara Tybur's War Hammer Titan, but unfortunately, its powers protected it from the worst effects of the thunder spears, and it was up to Eren to finish the job and steal Lara's powers for himself.
Most recently in the anime, Levi Ackerman used several thunder spears to destroy Zeke Yeager's Beast Titan and capture Zeke, and Levi even took the extra step of embedding one last thunder spear in his body. If Zeke struggled too much, he would accidentally yank out the thunder spear's activation cord and blow himself sky high. Now that's a creative use for Paradis Island's latest military innovation.