Redrafted this now that we have everything from the Fourth Databook and I'll be doing Orochimaru's Curse Seal later on too.
Juugo's Clan (
Official Version)
Once, the Tenbin Clan were a family affiliated with the Ryuuichi Cave and fought alongside the snakes as their allies and summoners. Possessing strong bodies and powerful chakra that was rumoured to be inherited from the Sage of Six Paths, several of their number in every generation would train in the art of senjutsu and become Snake Sages. They were even called Tenbin - Scales - because of the balance of energy needed to use senjutsu.
But one of their number grew too greedy for power, desiring a means of giving senjutsu to all of his clan without the dangers. He sacrificed the snakes that were meant to be his allies to unravel the secrets of senjutsu. And in the end, he did succeed.
It was a very unique and very powerful fuuinjutsu.
This fuuinjutsu absorbed natural energy and then fed it into the bearer's body, which they mix with their physical and spiritual energy to produce sage chakra. It was just like when the snakes fused their bodies with a Sage to gather natural energy in their place, removing the need for them to remain still during battle. And as a failsafe, it would protect the bearer by preventing an imbalance of energies within their bodies from transforming them into statues of stone.
Soon enough, everyone in the clan was branded with it.
However their actions earned the entire clan the hatred of the snakes that had been betrayed. The snakes turned against the Tenbin with fangs and venom and broke the contract between them. Without their connection to the snakes through that contract, their senjutsu was no longer influenced by them and took on an entirely different form. Instead of gaining the properties of snakes when they entered Sage Mode, they were horrified to discover that they would transform into entities that resembled monsters and demons.
Worst of all, the snakes used the generations of their clan's blood that had used to sign their summoning contract to cast a curse jutsu upon the entire Tenbin Clan. That curse twisted their new seal into a curse seal which corrupted their bodies and minds. It would still draw in natural energy and protect them from petrification, but now it could not be shut off and when they inevitably overdosed on nature energy they would be transformed into homicidal demons for a time. Only by becoming a Perfect Sage could a Tenbin avoid this fate, and precious few people ever achieve that level of skill with senjutsu.
This should have been the downfall of the Tenbin Clan; they had lost the source of their senjutsu knowledge, their snake jutsu and summons and now suffered from berserker rages. But the tool that doomed them was also what saved them. It held a terrible cost but was considered worth the risk and and they called this new power Sage Transformation.
It wasn't until the next generation that a key factor of the curse seal was discovered. For better or worse, it could be inherited.
Maybe because the Tenbin were descended from the Sage of Six Paths, because they had so deeply adapted to senjutsu after all this time or because of the curse itself, its power was passed down from parent to child like an artificial Kekkei Genkai. Children who hadn't started any kind of senjutsu training would absorb natural energy through the curse seal and suffer temporary bouts of insanity that could cause devastating amounts of destruction. But despite how disturbing this event was, it was soon seen as a blessing.
The Tenbin Clan started training in their branch of senjutsu from the youngest ages, something previously impossible because of the strength needed even to learn it, and they soon developed into a terrifying fighting force. Even unable to master senjutsu so quickly, their violent rampages still made them a deadly threat against enemy clans that only grew moreso with age and experience.
And this early training taught them to transform their bodies in ways that the previous generations never would have conceived, no longer bound by the serpentine form of a Snake Sage. They learned to alter the very shapes of their cells to turn their limbs into weapon, steal or transplants their own and fire destructive waves of sage chakra. They were feared as monsters in the shinobi world.
It was during the Warring States Era that the Tenbin Clan fell, their enemies joining against them in a temporary alliance to remove their threat. They were survivors but their berserker nature made hiding difficult as they were draw hatred wherever they travelled.
Today, all that's left of the clan is a single boy known as Juugo of the Scales to warn others of his true nature and of his heritage.