Shadow Steel (陰鋼 Intetsu) is a powerful Secret Treasure created by the Dark Elves for the purpose of defeating their enemies. According to Iscariot, it was created approximately 1000 years ago, making it one of the oldest of its kind.
Intetsu is an S-Class Secret Treasure with unique properties that make it one of a kind. Unlike most other treasures, it does not require its user to pay a heavy price beyond their own magical power. While not in use, Intetsu remains into a semi-solid cubical form that can easily fit within the average human palm. It also, by itself, does not possess any magic, which also makes it easy to conceal from potential enemies. However, Intetsu is highly sensitive to outside magical stimuli, which can potentialy increase the effects of its user's magic that they channel through it by roughly tenfold. It can make a normally powerful spell into something abolutely destructive. As such, one of the drawbacks of using Intetsu is that, if mishandled by an inexperienced mage, it may not only hurt or even kill them, but also everyone in the nearest vicinity.
The most important ability of Intetsu is that it can manipulate its state of matter into anything that its user can imagine. From solid to liquid to even gas and back again, provided they have the sufficient magical power necessary to conjure up said transformation, as well as maintain it for extended periods of time, anything is possible for Intetsu. For the most part, this makes it indestructible and it can even withstand some of the more powerful spells used against it. Should there be magic capable of "destroying" its body, Intetsu can easily reform back into any state chosen by its master, by simple application of the necessary magical energy. Although it can theoretically shape into even an armor or a large and heavy weapon, should its user lack the needed magical energy to manage such a form, it can potentially kill them. That is the main reason it is classified within the highest possible rank of Secret Treasures.
Advanced Armament (進武装, Shinbusō): The offensive capabilities of Intetsu.