Uber Saiyan

Uber Saiyan is a Saiyan transformation in which the user reaches the maximum potential of a Super Saiyan. This form causes the Saiyan to experience extreme conditions such as enhanced senses, mobility and power. Furthermore, using this form will drain the user's energy fast, so this form will only be necessary in short periods of time. Gohan first experiences this form during a fight with God Cell (An alternative version of Cell created using Cell's original DNA plus DNA from Beerus).

At this point, Gohan has turned Super Saiyan God himself and transforms Uber when Cell reminds Gohan of the pain and suffering he put the rest of the Z-fighters through last time they fought (God Cell has the same brain as the past Cell, making him basically a resurrected, stronger version of Cell).

Gohan becomes enraged and imagines the lives of Videl and Pan getting annihilated. He becomes fragile while transforming as he's focusing all energy on this transformation allowing Cell to strike him. With the help of Goku and Vegeta, Gohan manages to transform. After the fight, Whis explains that Gohan was able to transform into such a state because the saiyans he used to transform into a god were already gods (2 gods, Goku and Vegeta) and the hidden potential locked deep into him. Uber Saiyan mixes Super Saiyan God with Super Saiyan 2 (SSJ2 Hairstyle, Lavishing Red Hair and Sparks of Pure Energy).

Uber Saiyan first appears in the Canon film: Dragon Ball Z - Battle of Gods 3: Reinforced Madness set 6 months after Revival of F. Through these 6 months, Gohan decides to train once again as he realises there are new and old threats that can come back and threaten the earth again. Whis also tells Gohan that he has huge potential and decides to train him alongside Goku and Vegeta. During the film, you see Gohan transform into a Super Saiyan God with the help of Vegeta, Goku, Goten, Trunks and Future Trunks.

Power:

During the fight with Mega Cell, who is stronger than Beerus, Gohan finds himself toying with Mega Cell while in this transformation but later shows that he's not used to the power this form possesses. Once a Saiyan has perfected this form then it may be the strongest form in the universe. Gohan only uses this form in a four minute fight (Form depletes during fight with Cell, Gohan was lucky enough to drastically weaken Cell enough so he can finish him off with the help of Goku and Vegeta using the Spirit Kamehameha and Final Fragment).

From what we can see, the aura of the form will cause the atmosphere to erupt around the user. Trees, Earth, Rocks, Clouds and even close buildings and planets start to rumble and break, this cannot be controlled. Futhermore, this suggests that Uber Saiyan requires a body that can sustain infinite energy which is impossible therefore this form isn't compatible enough and is too powerful which can cause damaging effects on the Saiyan.

Whis tells that this form hasn't been seen before but was in an ancient prophecy about a warrior who had incredible power, a power that was too strong for the user himself, and later, the form ended up tearing a rip into the galaxy, causing it to implode, killing the warrior and the entire galaxy's population. Though, this was just a myth.

Energy blasts from Gohan couldn't be sustained, he had trouble controlling them. Instead, he had to stick to basic combat. Further into the fight, Gohan began uses a mixture of energy blasts in his punches, this is explained later by Piccolo that the form is trying to rid itself of energy.

Gohan later learns to control this form a bit better and can now produce ki blasts.

Perfecting the form:

Whis ends up training Gohan in this form with the help of Goku and Piccolo. After a year of personal training with Whis, Gohan arrives to planet Earth once again with control of Uber Saiyan. Now he is able to last a short period of time longer and is capable of using devistating energy attacks that doesn't affect his stamina. Although he has learnt this, Whis has told him to not use this form unless it's a last stand. Otherwise, the planet is in risk of destruction.

King Kai predicts that this form can never be perfected. It would require thousands of years of training if it was possible.

Uber Saiyan Users:

Gohan

Goten

Trunks