This ability allows the user to materialise their will into a physical form, appearing as a black substance that can be used for various purposes.
Immediately prior to being sealed by her two sons, Kaguya Ōtsutsuki formulated a plan that would result in her eventual revival. To accomplish this, she created a sentient being known as Black Zetsu out of her will, which would enact this plan in her absence. As a malleable construct of Kaguya's will, Black Zetsu is capable of controlling the actions of others by coating itself over their bodies and taking possession of them.
Madara Uchiha was also capable of using this ability. Moments prior to his death, he attempted to infuse his will into the right half of White Zetsu, seemingly creating Black Zetsu, which he referred to as a weaker version of himself; in actuality, Black Zetsu merely made it look like it was being created as part of its plan. Madara then caused a black rod to emerge from the flowering tree he was attached to and told Obito it was a physical manifestation of his will, and that he should use it when performing the Six Paths Technique.
On a later occasion, following his resurrection, Madara caused a number of these black rods to emerge from Obito's artificial right half, which became entirely coated in his will. This allowed Madara to remotely control Obito's actions using his chakra, as well as fire black rods out of his body as projectiles, though Obito was ultimately able to resist his control.