Dragon Ball has long featured fighters who physically transform into even stronger states and, as the franchise's main protagonist, Goku has more transformations than any other character in the long-running manga and anime series. Dragon Ball Super saw the Saiyan martial artist attain his most esoterically powerful form yet: the Ultra Instinct state.
First unveiled during the anime series' final story arc, "The Tournament of Power," the transformation is apparently not a conscious one. It's only emerged during moments of extreme duress, and it causes Goku to surpass his own natural limits even as a Super Saiyan Blue using the Kaio Ken technique. The form is first seen incomplete and in a defensive capacity as Goku fights the Universe 11 fighters, Top and Dyspo, as well as Universe 6's fused Super Saiyan warrior, Kefla.
What Is Goku's Ultra Instinct Form?
After reflexively transforming into the imperfect Ultra Instinct state, Goku gained a silvery blue aura with his hair reverting to its natural black color with flecks of silver that matched his now-platinum eyes. His strength and speed also significantly increased. In this form, Goku was able to surprise and defeat his physically superior opponents. His divine teachers, Whis and Beerus, theorized that training with gods for an extended period of time gave Goku the ability to tap into god-like powers in short bursts.
While the form was dubbed "Ultra Instinct" by the two deities, Goku is unable to sustain the form for extended periods, since the physical stress of pushing his body above its natural limits causes his blood to burst and leaves him largely incapacitated while he recovers from enduring such a strenuous transformation. By the end of the Tournament, Goku gained a greater mastery over the form, with his silvery aura appearing more stable. His hair is now completely platinum as he faces Universe 11's champion, Jiren, in the final bout of the multiversal martial arts competition. In this mastered, complete state, Goku appears much calmer and in control of his new form and boasts previously unseen power levels that allow him to outmatch Jiren for a time.
However, just as with the incomplete Ultra Instinct form, the mastered, silver transformation exacts a heavy toll on Goku's body. It left him severely weakened after its prolonged usage, right before Goku could deliver the finishing blow on his opponent. In a way, the Ultra Instinct form operates like a combination of adrenaline and the franchise's Kaio Ken technique. Even after undergoing the complete transformation, Goku is unable to trigger Ultra Instinct at will, and he didn't use the form during his fight with the titular Saiyan warrior of Dragon Ball Super: Broly.
Ultra Instinct Isn't Goku's Most Powerful Form
In the manga, which has continued past the events of the anime series, Goku has admitted that he's been unable to achieve his Ultra Instinct form in the years following the Tournament of Power. This suggests that the transformation is like adrenaline and only occurs when Goku is in moments of physical duress that force him to tap into abilities beyond his natural capacity to survive. Like Kaio Ken, a divine technique taught by King Kai at the beginning of Dragon Ball Z, the temporary power boost does take a visible toll on its user's body, running the very real risk of grievous injury in a desperate bid for added strength.
While Goku is the only one seen using Ultra Instinct, it is implied that Whis and Beerus have similar abilities, given their divine combat nature and their ability to recognize it when Goku first undergoes the transformation. What's particularly interesting is that Goku's Ultra Instinct state does not appear to be explicitly tied to fighters of Saiyan origin -- it can be achieved by any fighter that trains with the gods. In the manga, Roshi is theorized be the human most capable of reaching the form after centuries of martial arts mastery. And given Vegeta's time training with Whis and Beerus, Goku might not be the only Saiyan who's able to reach Ultra Instinct form before long.
While it was the most powerful transformation seen in the Dragon Ball franchise at the time, an improved version of Ultra Instinct was introduced in Dragon Ball Super chapter 85: True Ultra Instinct. The new form subverts all the previous rules about Ultra Instinct, but is Goku's most powerful form yet as he's mastered the divine technique of Ultra Instinct, as artist Toyotarou has claimed. In this state, he could indisputably rival Beerus and Whis in terms of raw power and stand as one of the most powerful fighters in all of Dragon Ball.