Jūgo's Clan's Kekkei Genkai

Members of this unnamed clan have unique bodies, which come with two complementary, but distinct traits.

Natural Energy Absorption
One of them is a passive absorption of natural energy regardless of any input or consent whatsoever, providing them with various benefits, like bypassing the need to stand still, among others. Thanks to this, they have potentially a much easier time to use Senjutsu. However, it also comes with adverse effects of its own, like mental instability in a form of sporadic, sudden and uncontrollable surges of rage, madness and an urge to kill.

Sage Transformation
As the second, their bodies also produce special fluids, which respond to Senjutsu Chakra when molded by affecting the bodies in a way that transforms them and and allows to drastically alter shape. Common traits include change of skin color and yellow eyes, otherwise the form and extent of the transformation depend on the users.

Background
Having once lost control of himself and massacred a whole village, Jūgo secluded himself from crowded places, living in caves to prevent more harm be done to others in case it happens again. That was until a young boy visited him one day, with an offer he could hardly refuse. Jūgo was given a promise of cure if he follows Kimimaro to Orochimaru's place. He readily accepted and allowed himself to be experimented upon, unbeknown of Orochimaru's true intents, who was more interested to find the source behind Jūgo's transformations rather than helping cure his unstable mind. Further research led to the discovery of Ryūchi Cave and the development of Curse Marks.

Legacy
After the death of his former master and partner Orochimaru, Kabuto Yakushi had lost a purpose in life and felt lost. Having had Orochimaru's words about finding one's own true self strongly embedded into himself, he along the way on this quest to attain perfection had injected himself with cells of various people and as a result has gotten access to their powers. Among them were also those of Jūgo, granting himself both the ability to passively absorb natural energy and having his body create the same bodily fluids which allow the use of Sage Transformation. Thanks to this, he could master Senjutsu at the Ryūchi Cave more easily.

Trivia

 * Unlike ordinary individuals, the danger of turning into an animal and eventual petrification in case of failure to balance natural energy with chakra doesn't seem to be of concern to members of this clan or those with their powers.