The tool itself is rather big and looks more like a sword made out of a dark yellow metal. In the middle-section of the Key there is a set of three teeth set symmetrically on each side. The "teeth seem to be in the shape of the Japanese kanji 王 Ou which means 'King' or representing a "E" for Eibon.
The purpose of the demon tool is to activate BREW. Without it, BREW will not activate, no matter what one does. Its true power resides in the fact that it contain's Eibon's soul, Eibon being the only person who can activate BREW. Since Eibon has bound his soul to the demon tool, Eibon shares a sort of symbiotic relationship with the Demon Tool. The Key is set to disappear after activating BREW, and Eibon disappears alongside it.
Shortly after the creation of BREW with the help of Arachne, Eibon realized how powerful the Demon Tool was, and feared the chance of it falling into the wrong hands. In order to make sure BREW could not be used just by anyone, he created his last Demon Tool, the Key, and bound his own soul to it, sealing away the only person who can activate BREW.
He sealed the Key in a relic deep underground. He also created the Clowns as its guardians, if anyone unearthed the Demon Tool. He programmed the Clowns to state a message from him if they were defeated, warning the person who acquired the Key to use the Demon Tool wisely.
The area around the underground relic soon became the town of Sarcophagus. The people of Sarcophagus discovered the relic, which caused the Clowns to activate and devour everyone in the peaceful town. Thus Sarcophagus was reduced to a wasteland, with the Clowns devouring anyone who entered it.
At first, Kid goes inside Sarcophagus alone, leaving behind Liz and Patty for the reason of safety. He soon discovers the relic and finds the Key, but is attacked by the Clowns. He struggles to escape on his own, but Patty and Liz come along to help. With their efforts, they defeat the Clowns using Death Cannon. As Kid leaves, one of the Clowns plays Eibon's last message, warning him of using the Demon Tool, which causes Kid to doubt the DWMA's actions.
After returning to DWMA, Death the Kid does not relinquish the Key at first, asking Shinigami what he intends to use it for. To his surprise, Shinigami and Joe Buttataki happily comply to his demands. They explain that the Key is Eibon himself. Eibon appears from the Demon Tool.
After Excalibur and Shinigami explain to Death the Kid about Eibon's past, Eibon agrees to activate BREW for Shinigami in order to make amends for betraying them and working alongside Witches. He inserts the Key into BREW's true form and activates it using his own soul. After activation, both the Key and Eibon disappear.
- ↑ Soul Eater Episode 47 (Funamation Sub) — Death: Actually, this is Eibon's creation...the Magic Tool Eibon.