The ANBU Handbook
In many ways, Science and Seals are very much alike. Each are capable of many great yet terrible things. Yet, perhaps fortunately, neither of these arts are at all well known.
At any given time, there are perhaps half a dozen masters of either. The most well known Seal masters of the time, Orochimaru of the Hidden Sound and Jirayia along with his student the Fourth Hokage, Namikaze Minato, had all achieved Master status by the age of 17, and all originally hailing from Konohahakure.
One of the greatest examples of sealing in any of the elemental nations was executed by Namikaze Minato in the year 127 AS(After Sage{The Six Realm Sage is attributed with the birth of modern Jutsu along with the creation of Soldier Pills and the use of seals as more than a dead language from a forgotten Era.}). The 'Dead Reaper Seal' was developed as a means of containing any malevolent entity. In theory, anyone or thing can in turn be sealed into anyone or thing. This seal was used for the first and only time by the Yondaime Hokage, sealing a collective of negative energy and emotion, known as the Kyuubi, into Minatos day old child. The 'Demon' energy was absorbed into the child's Chakra system, and slowly yet surely mixed with it to make it more potent yet harder to control at the same time. This same child also developed 3 whisker like marks on each cheek as an identifier of his Jinchuriki status.
On the flip side though, would be Orochimaru's 'Heaven and Earth' Curse Seal, known to have at least 2 different, yet complete levels, possibly even more. This seal allows the gradual mutation of the subjects Chakra network. Specifically around the brain. It allows the creation of hormones and chemicals that increase the combat abilities and aggression, yet also surrenders mental faculties to the giver of the seal which, in all known cases, is Orochimaru.
If you ever have to fight a Seal user at ANY level of mastery, do NOT let them choose the battle field. If given ANY time to prepare, a Genin Seal user can over power a Kage level Shinobi easily. In some countries, sealing is also known as voodoo or witchcraft and hold the death penalty to any practitioner. This is one of the reasons there are so few left among us.
In short, we should all be grateful yet fearful for the small number of 'Masters' who live among us.
-Excerpt from the 'ANBU Handbook' – Chapter 1- 'Seals and Science'
