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 Demon Tool was created by both Eibon and Death, having signed their signatures after it's creation. At some point a century prior, it fell into human hands and was then attached to the Runaway Express and ran for 100 years after. The Runaway Express became a legend, with those mostly unaware of it's power source.
The Demon Tool fell into DWMA hands when Death the Kid, Liz, Patty Sid Barrett, and Mira Naigus secured it from Arachnophobia and Medusa's Army. It was then later used to power the Demon Airship. In the anime, it is instead used to power the Death City Robot.
The Eternal Spring is a powerful Demon Tool, capable being an infinite power source for a given machinery and also preserves the machine by attaching the box to a machine and setting the key inside the keyhole.
- ↑ 1.0 1.1 1.2 Soul Eater Manga: Chapter 31
- ↑ 2.0 2.1 Soul Eater Manga: Chapter 30
- ↑ Soul Eater Manga: Chapter 90
- ↑ Soul Eater Anime: Episode 47