Orochimaru's Cursed Seal v1 (
Version 2)
Long before Orochimaru ever left the Hidden Leaf, he learned about an extinct clan from the Warring States Era capable of bizarre bodily transformations. Like all such abilities, the idea fascinated him and he resolved to look deeper into them.
His investigations taught him that they possessed some type of genetic cursed seal which fueled their transformations. When he looked into it, he found its source in the Ryuuichi Cave, the homeland of the summon snakes. It was here that Orochimaru first signed the snake summoning contract just as Jiraiya found the toads in Mount Myoboku and Tsunade found the slugs in Shikkotsu Forest.
And it was here that Orochimaru tried to learn senjutsu, the original source of the Tenbin Clan's power. He made some progress in creating sage chakra but his body simply wasn't strong enough to withstand the rigors of harnessing it and thus couldn't bring out its true potential.
This is partially responsible for why Orochimaru sought immortality by transferring from one body to another as he believed that a more powerful body would allow him to achieve a true Sage Mode. Until that time, he decided to developed an alternative method of harnessing senjutsu based on Tenbin's power; a cursed seal that would induce an artificial type of Sage Mode in the subject.
The cursed seal was made from Orochimaru's sage chakra and a fraction of his consciousness resides within it when he brands the subject. It works by drawing the individual's chakra into the cursed seal, converting into Orochimaru's sage chakra - molded by the fragment of his consciousness inside it - and then pumping back into their body. The more senjutsu chakra is produced, the further the cursed seal spreads over their body and the more powerful they become. And with Orochimaru's chakra flowing through inside them, the bearer's darker emotions and impulses are stimulated whenever they use the cursed seal.
Each cursed seal manifests in a design unique to the individual. There are only two patterns known to reoccur which would appear in the hosts with the greatest power or potential. The Cursed Seal of Heaven for those attuned to Yin, and the Cursed Seal of Earth for those attuned to Yang.
After the branding, the victim falls unconscious as their body is forced to adapt to its tainted power. Not everything survives and it was proven to have a one out of ten survival rate when Mitarashi Anko, Orochimaru's favoured student, was the only one to live through the process during the first round of experimentation. That was one of the last experiments Orochimaru conducted as a shinobi of Konoha.
Anko's cursed seal was studied by Konoha but they could find no way to remove it without killing her. The only one who might have been able to divine more about its nature was Jiraiya but he was preoccupied in chasing after Orochimaru who had defected. Fortunately, Anko never used its power which kept her free of its influence until it was suppressed and dormant.
It wasn't until years later Orochimaru that he finally got his hands on a living member of the Tenbin Clan for his experiments. He extracted the Juugo's bodily fluids and refined an enzyme that induced similar powers in the subjects of his cursed seal.
This new version of the cursed seal is injected into the target rather than just branded on them. On the surface, it appears to be no different from the prototype cursed seal which is what misled those who saw Sasuke's cursed seal as they assumed it was the same as Anko's. In actuality, this version has two levels.
Level 1 functions the same as the prototype for the most part. But the enzyme means that its mental influence is far greater before so that it cannot easily be suppressed - reflecting the volatile nature of the Tenbin Clan - and it has the potential for growth. The more the cursed seal is used, the more it will invade the body and mind until they are completely taken over. Once that level of corruption is reached, the bearer will have lost all free will to Orochimaru and will achieve the next plateau of power known as Level 2 or the Cursed Seal Transformation.
This state can also be induced by the Mind Awakening Pill, a drug that Orochimaru developed that increases the speed of the cursed seal's contamination to forcefully raise it from Level 1 to Level 2. This is a lethal process since the body cannot safely adjust to that sudden influx of power, so it requires the use of the Four Blacks Fogs Formation and Dark Sealing Method to soften that death to a temporary coma until the drug's work is done.
This method can actually circumvent the brainwashing effect of the cursed seal because it doesn't have the time to corrupt the bearer. Sasuke was partially able to preserve his own mind because Orochimaru's desperation pushed him to evolve the boy's cursed seal early to make him a suitable vessel. With the powerful bloodline of the Uchiha and a full evolved cursed seal, Sasuke would have been the perfect container.
This Level 2 is where the cursed seal truly mimics the powers of the Tenbin Clan. It draws natural energy directly from the world to empower the bearer instead of using Orochimaru's sage chakra. This induces a transformation similar to Sage Mode where their bodies take on monstrous appearances and all of their abilities are amplified by the cursed seal chakra. Their strength, speed, durability and stamina increases, the power of their jutsu is heightened and even Kekkei Genkai are pushed beyond their usual limits. The only cost is that using Level 2 causes a strain on the body that can leave them unnaturally exhausted once it's released. Those particularly skilled in using the cursed seal can undergo partial transformations instead, taking advantage of their altered forms without the strain.
Orochimaru's ideal vessel would have possessed a naturally strong body that comes from a powerful bloodline such as the Kaguya or the Uchiha, and would have been corrupted by the full evolved cursed seal to provide a body capable of using senjutsu and remove any resistance against the Living Corpse Reincarnation. But Kimimaro's sickness and Sasuke's defiance prevented that ideal from ever being made a reality.