The Eternal Spring (永久弾機, Eikyū Zenmai) is a Demon Tool that is created by Eibon. As the name says, it is capable of letting a machine run on for eternity without fuel or maintenance. It provides movement for a steam train, making it run across the Sahara desert without fuel or maintenance for a hundred years. The Fisher King, Mizune and Death the Kid fight over it, with Kid victorious in the end. DWMA carpenter Gen later adapts the Eternal Spring for use no longer in a train but to power the Demon Airship to take DWMA forces to confront Asura on the Moon.
|“||This is an evil Tool. Just holding it disgusts me.||”|
—Death the Kid, Episode 30
The Eternal Spring consists of two parts. One is of a large ornate key-like object (about ten times larger than an average key) similar to the ones used in clockwork toys. It seems to be made of a brass-like metal and has magic inscriptions going down it.
The second part of the Demon Tool is the big red box with a keyhole in which the key goes into. The box has pipes coming out of it so it can attach itself to a machine. The key turns slowly on its own inside the box, causing the machine that the box is attached to, to undergo the effects of the Demon Tool and move on its own accord exactly in the manner of a clockwork toy. If one is to open the box, a plate with an inscription can be found among the wires, made up of both Shinigami's and Eibon's signatures.
The function of the Eternal Spring is to act as an alternate fuel source. By attaching the box to a machine and setting the key inside the keyhole, the Demon Tool provides energy in order to activate the machine. The machine can go on working without any worry of running out of power or fuel, and needs no maintenance.
However, it is shown to have an immense amount of power, up until the point that if the Demon Tool is not deactivated, the machine will continue running on its own, like the Runaway Express. The Eternal Spring proves to be almost too powerful for simple man-made machines like a steam train, making the train run on for a century with it never stopping and speeding through the desert at incredible speed, even with no railways.
It also seems that the Eternal Spring preserves the machine it is attached to as the Runaway Express seems to be in perfect condition despite the long period of time.
By the inscriptions in the box, the Eternal Spring was a joint effort made by Shinigami and Eibon. By means unknown, it fell into human hands and ended up as an engine in a steam train a century prior to the series. The result was that the train went berserk, and ran on its own at great speed through the Sahara desert. The train thus became famous, being known as the Runaway Express.
Part in the Story
Upon reaching the station in which the Runaway Express (the steam train running berserk because of the Demon Tool) is set to arrive at, they meet a disguised Mizune using Soul Protect. Soon, the Runaway Express arrives spectacularly from underground. One of Arachne's agents, the Fisher King appears and boards the train first. Mizune attacks Kid and flies in pursuit. Kid is not far away with Liz and Pattyon his skateboard.
After a high-speed chase, Kid manages to enter the train and fights the Fisher King, whilst Mizune attacks from above. In the end, Kid manages to get the Demon Tool first. He pulls the key out of the box, stopping the train. He expresses disgust at the Demon Tool, classing it as evil. The Fisher King tells Kid that the Demon Tools are a work of genius, created by Eibon. Death the Kid responds that Eibon is evil, and is angered when the Fisher King tells him that Eibon and Shinigami are the same. He tells Kid to open the box of the Eternal Spring. He does so and finds to his shock, Shinigami's and Eibon's signatures. Before the Fisher King can tell more, he is killed by Sid and Nygus, who take the Demon Tool as Mizune leaves. Death the Kid ponders on what the Fisher King has told him.
DWMA carpenter and engineer Gen installs the Eternal Spring as part of the configuration for the new Demon Airship to more effectively fly Franken Stein, Death the Kid, and the Death Scythes to take down the Kishin.
- ↑ Soul Eater Manga: Chapter 91