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.

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. Hringhorni 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, eliminating many VIRM armadas and sacrificing their lives to destroy the VIRM planet.

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.