Hringhorni

Hringhorni is a top-secret klaxosaur weapon in DARLING in the FRANXX. It is an important construction project closely supervised by Papa and the Sages, who refer to it as the weapon that will liberate humanity and wipe out the klaxosaurs entirely. Dr. FRANXX refers to it as the ultimate weapon.

Appearance
Hringhorni is a massive spaceship, it resembles a giant spear. Under VIRM's control, it is mostly purple with magenta lines. When Strelizia Apus connects to Hringhorni, its colour changes to white and it has red and orange lines similar to that design of Strelizia.

In the manga, rather than a spear-like weapon, it is a giant colossal Klaxosaur weapon and takes on the appearance of a massive colossal Klaxosaur baby like creature. Once merged with Strelizia (anime) or a 9 Model (manga), it takes on a FRANXX appearance larger than other FRANXX. It does not have a face that expresses its pistil's emotion, and its helm is larger. It possesses four legs, has wing-like structures on its lower back, and a laser weapon on its forehead.

Background
APE had been making preparations for a weapon powerful enough to eliminate the entire Klaxosaur population but lacked the resources to build one. As destroying the cores of Klaxosaurs are necessary to kill a Klaxosaur, and they are essential for the weapons humans use (which everyone outside of APE knows are actually Klaxosaur weapons), Papa has had his sights set on obtaining the Gran Crevasse, which is the location where 001 resides, as it holds a significant amount of Klaxosaurs and could obtain their cores to build the weapon. After the successful siege of the Gran Crevasse, the area was used to begin the construction of Hringhorni.

On the contrary, in the manga, Dr. FRANXX explains Hringhorni is an aggregation of Klaxosaurs after eons of fusion and evolution.

Episode 20: A New World
Dr. FRANXX describes it as "a massive spear built using klaxosaur cores". Inside it is a massive weapon called the ‘Star Entity‘. Papa says construction on it is almost complete and it will save humanity. Hiro and Zero Two are tasked with taking control of it.

Episode 21: For You, My Love
The Klaxosaur Princess, after forcing Hiro to link with her, attempts to take control of it but she is infected with a virus planted as a trap by Papa. Dr. FRANXX tells Zero Two that she was created to operate it on behalf of the human race. Star Entity is revealed to have been implanted with a self-destruction bomb by Papa that will destroy the entire planet. However Hiro and Zero Two stop it and Papa, along with VIRM, takes Hringhorni with them as they flee.

Episode 24: Never Let Me Go
VIRM uses Hringhorni against Hiro and Zero Two to severe their connection but they reconnect and take control of it, converting back to its original state. They use it to eliminate many VIRM armadas and sacrifice their lives to destroy the VIRM planet. Hringhorni is presumed to be destroyed following the explosion of the klaxosaur bomb.

Etymology
In Norse mythology, Hringhorni is the name of the ship of the god Baldr.

Trivia
   
 * Although publicly stated to be the tool to humanity’s survival, it was actually built as a last resort to not only eliminate the klaxosaurs but to destroy the planet in the process.
 * In the manga, 001 uses Hringhorni to destroy all remaining Plantations, including Plantation 13.