0 Series

The 0 Series is a mysterious series of weapon unlocked in the Legendary and Vassal Weapons. They are effective against "those who possess eternity" and are hypothesized to be left behind by the God Hunters.

Overview
By absorbing a drop of Red Poison, which can be found in a Guardian's base like Sanctuary or the Ancient Labyrinth Library, the 0 series is unlocked. This form of weapon has 0 in every stat while the accompanying skill "0" has zero cool-down time and consumes zero SP. This seemingly useless shield sacrifices all ability to work on normal creatures to be an effective attack and defence against the self-proclaimed gods.

Chronology
The Red Poison was first found in the depths of the Ancient Labyrinth Library. After absorbing it into Kizuna's Legendary Hunting Tool, she unlocked a weapon called "0". Later on, she is able to use it to expel the doping powder that the Vanguard of the Wave Seya was using. Realizing that they could use this series of weapon to "repel the connections formed by illegitimate power, the Heroes then test this weapon against the crack in the sky caused by a wave. Almost immediately, the wave started to recede and the time until the next wave was longer than usual.

Returning back to the world he was summoned to, Naofumi started the search for more of the Red Poison to unlock the 0 Series for himself. With the help of Fitoria, he was able to find it at the heart of Sanctuary. He, and all the heroes present unlocked the 0 Series weapon for themselves. The room containing the Red Poison also had murals depicting the Legendary and Vassal Weapons as well as a cat-like entity. According to the mural, the 0 Series is effective against those who "possess eternity" or the self-proclaimed gods. With these weapons, the heroes are supposed to act as "stopgaps" until true help can arrive. Later on, Naofumi unlocked the Dragon King Shield 0 which had the equip bonus called "Beginning of 0".

While in the ancient past, Naofumi and his party encountered a self-proclaimed god during a wave. The self-proclaimed god nicknamed "Cathead" easily overpowered the entire group. However, the Legendary Shield pointed Naofumi to the 0 Series and it skill. Suddenly he was able to block Cathead's attacks. Surprising Cathead, the heroes switched to their 0 Series weapons and killed him.

Later on, Naofumi's allies were inflicted with "incurable" wounds by the self-proclaimed gods. Naofumi promised himself that he would do anything to save his allies. Responding to his conviction, a voice asked him if he was truly willing to go down the thorny path. Responding yes, the Beginning of 0 status was changed to 0 Territory:Upper Layer. This reduced Naofumi's attack stat and taught him to cast 0 Series magic. With this, Naofumi was able to reverse the "incurable" wound at the cost of all of his magic power. Beside the 0 Territory skill appeared the value 3%. After casting this a few times, this value increased to 7%.

Powers
So far, there have been two ways to unlock effects from the zero series: unlocking the 0 Series weapon, and advancing the 0 Territory skill/status.

Weapon Forms
By absorbing the red poison, the first weapon of the 0 Series is unlocked. The name of this weapon is always of the form "[Weapon Name] 0". As of Volume 22, the Legendary Shield is the only known weapon that has advanced past the first weapon of the series. The known weapon forms can be seen below:

0 Territory
After unlocking the Dragon King Shield 0, Naofumi unlocked the equip bonus called "Beginning of 0". This seems to be the start of the 0 Territory progression. After Naofumi's allies were attacked by weapons created by the self-proclaimed gods, he heard a voice in his head asking if he is willing to go down the thorny path to resist them. After confirming that he was, the effect "Beginning of 0" was changed to "0 Territory: Upper Layer". It is unknown if these effects will change depending on the weapon.

Magic
Upon changing his 0 Series status to "0 Territory:Upper Layer", Naofumi learned how to use 0 Series magic. This unique magic is the hardest magic to use by far and is unknown to the Demon Dragon Emperor. Naofumi compares it to scooping fish out of a pond with a flimsy membrane - like wet paper.
 * First Heal Zero: Heals wounds which have been afflicted by the self-proclaimed gods. Where normal healing methods would be reversed to make the injury worse, this spell removes the "curse" altogether and allows for proper healing.

Trivia

 * Holn hypothesizes that the power to unlock the 0 Series was left behind by God Hunters for future generations.
 * Possessing a 0 Series weapon provides a passive defence against long-distance scrying by the self-proclaimed gods.
 * The 0 Series also seems to be effective against dragons since they have their own form of immortality.