Jūgo's Clan's Kekkei Genkai

The kekkei genkai of Jūgo's clan is an instinctual ability which allows its members to absorb and make use of natural energy without any training in senjutsu.

Overview
Due to the steady but constant intake of natural energy, they are prone to mental instability, sporadically feeling sudden and uncontrollable urges to kill. The wielder's body can also secrete special fluids which, through means of Senjutsu chakra, allow them to transform parts of their bodies. While varied by appearance and function from user to user and even from use to use, the affected parts of their skin change colour (the hair colour also may change) and if the transformation occurs in the eyes, they change colour as well. Seemingly, the risk of petrification from the imbalance of natural energy, appears to be non-existent.

Being that the natural energy gathered can temporarily be balanced within the wielder of this ability, they can safely transfer their accumulated senjutsu chakra to another individual.

Background
After Jūgo massacred an entire village during one of his rages, he secluded himself, living in caves, until he was approached by Kimimaro, who told him Orochimaru could help him with his urges. Jūgo went to Orochimaru voluntarily, and allowed him to research his transformations. More interested in the source of the transformation than in curing the killing urges, Orochimaru experimented with Jūgo's body fluids, developing an enzyme that can trigger similar changes in other, suitable hosts, through cursed seals, which Orochimaru determined to be ten percent of the population, all others dying after having the cursed seal applied. However, their transformations are limited to fixed forms, able only to transform into a specific form back and forth. His investigations on the source of their power also led him to the discovery of Ryūchi Cave.

After Sasuke absorbed Orochimaru, Kabuto Yakushi found himself without purpose, and following Orochimaru's belief of adding to himself to find his true self, injected himself with the biological material of the subjects of many of Orochimaru's experiments, including Jūgo. Doing his all to emulate his master, Kabuto went to the Ryūchi Cave, and the ability to absorb natural energy passively aided him in training senjutsu under the White Snake Sage.

In the anime, Tosaka did his own research on Jūgo's cursed seal powers, and duplicated it in combination with Scientific Ninja Tools to create a collar that can grant the wearer power similar to Sage Transformation. However, contrary to his claim of it being the "ultimate Cursed Seal weapon", Tosaka was completely overwhelmed by Jūgo's genuine powers.

Trivia

 * Despite Kabuto altering his DNA to obtain this kekkei genkai, he does not show any side effects of natural energy absorption like the clan's members. However, this could be the case simply due to Kabuto mastering Sage Mode.