The Thunder Spears are based off of technology that had been preserved in secret by the Interior Police, despite their orders to wipe out any sign of technological innovation. This technology was adapted to design a new special weapon for use against the Armored Titan. It works in a similar fashion as the Anti-Personnel omni-directional mobility gear's guns and hooks, but with a strong spear loaded with explosives designed to shatter armor.
These weapons work as a two-stage rocket: an initial charge launches the projectile by pressing a special trigger in the handle of the device. The projectile, connected by a rope to the main engine, is launched with enough power to penetrate hardened Titan skin. However, when that rope is cut, the second charge is activated. This causes a tremendous explosion which makes the projectile penetrate even more into the target.
Usage and limitations
The Thunder Spears are a weapon which enables a human team to efficiently launch a surprise attack against the Armored Titan. But this weapon plays the role of a double-edged sword, as it is a danger to its user as well as the Titan. So much so, that it is only useful in a specific type of situation where after use, immediate retreat to close structures via ODM gear is possible. Without a proper surrounding, the user is easily liable to get caught in the explosion. While it is also capable of damaging other types of Titans, it was designed solely to kill Reiner, the Armored Titan.
In preparation for defeating the Armored Titan, Squad Levi did not have much time to train, but Hange carefully placed their troops so that they would be in an optimal position to attack. It is a maneuver that requires three successive attacks: the first to blind the target, the second to break the protective shell, and the third one to kill.
Hange Zoë has the Corps of Engineers develop the Thunder Spears, using technology uncovered following the coup d'état. Designed as a weapon to combat the Armored Titan, Hange displays the performance of the spear to Scout Regiment soldiers.
In the battle in Shiganshina District, the members of Fourth Squad and the Special Operations Squad surround the Armored Titan in its battle, barraging it with explosives and blowing through its armor to detonate its nape.
Later in the battle, part of the Special Operations Squad combat the Colossal Titan. While Jean Kirschtein, Sasha Braus, and Conny Springer attempt to divert its attention, Mikasa Ackermann uses Thunder Spears to attempt to hit its nape from behind. However, the transparency of the plan leads to the attack being countered with a burst of steam. The shrapnel hits Mikasa, creating a shallow wound.
The same part of Squad Levi later use their four remaining Thunder Spears to combat Reiner, following his return to the conflict. Mikasa uses the first spear to break the Armored Titan's leg. Sasha and Conny try to use their spears to break Reiner's jaw, though one misses. Hange returns to the battle, using their spear to blow the second hinge of the Titan's jaw, allowing Mikasa to fire the final spear through its mouth, blowing Reiner directly out of his Titan's nape.