Orochimaru's cursed seals
Cursed seals are a form of power and control, granted by the Sound Ninja Orochimaru. They are usually a form of black dots or curved lines placed somewhere on the body, mainly either the neck or the shoulder. The seal grants the bearer power in the form of chakra; when releasing chakra the seal can be activated, and when active it draws out chakra from deep within the user.
Cursed seals were originally created by Orochimaru to help a ninja named Juugo who sought help from him. Orochimaru gave him one of his prototype cursed seals and Juugo entered a state of destruction. After that incident, Orochimaru continued to work on the seals until he eventually tweaked them just enough so that the bearer could control them. The downside however is the fact that only one out of ten people survive after obtaining a seal.
The cursed seal has two levels, level one and level two. Once receiving a cursed seal, one is only able to access level one, whereas level two is something that comes over time - either when the user has been affected by the cursed seal so much that they are able to access it, or if bearer drinks a Seishingan drug.
When a user activates level one of the cursed seal, the seal will glow and a black pattern will spread across at least half of the body. Level one allows the user to draw out more chakra and enhances their abilities. When a user activates level two of the seal a more dramatic change is experienced. The skin color will change and different parts will appear. These mostly include horns, wings, and tails.
Currently there are many different people in both the manga and anime that possess cursed seals, but the main ones include: Mitarashi Anko, Uchiha Sasuke, Tayuya, Kimimaru, Jirobou, Kidomaru, Sakon, and Ukon. In Mitarashi Anko's case, she has an inactive seal that doesn't appear to work, but it does show that she was "marked" by Orochimaru, hence to her training with him.
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