Brew, the tempest, is said to be the most powerful creation of Eibon. It is explained by Noah that BREW and The Book of Eibon are the wisdom of Eibon — essentially Eibon himself. It is currently in the possession of Crona in the manga's storyline, but in the anime it is in Death's possession.
This demon tool supposedly has many powerful abilities, one of which can greatly increase the soul wavelength of the user. It is also said that it has the power to raise storms. It was first used, unbidden, by Kid during his match with Mosquito. During that moment, it unlocked Kid's dormant powers — giving him enough strength to defeat Mosquito. Upon its activation, it gave off a response that was recognized by both Medusa and Noah.
In the anime, it had the ability to realize the user's own desires.
In the Anime, it is explained by Excalibur that Eibon had started researching immortality in order to save his sick wife, but he couldn’t make progress, and eventually turned to Arachne for assistance. The end product of the two’s combined research was Brew.
800 years before the start of the series, "Brew" was located in the Lost Island and was thought to have been destroyed by an accident that annihilated the entire facility and its population along with it. This was all caused by Arachne, who wanted to erase it from that era after she collected its blueprints. This, however, was not enough to destroy Eibon's greatest creation.
Part in the story
A significant battle occurred between the forces of Shibusen and Arachnophobia over the possession of Brew on the Lost Island. At first, it is thought that Arachnophobia won the battle but, ultimately, it was Medusa who managed to obtain "Brew". She had Eruka and the Mizune sisters take it, unnoticed during the battle and replaced it with a fake one.
"Brew" was given to Shibusen by Medusa as a way to gain their trust so she could work with them to take down Arachnophobia.