"I... I can't... control... it...I'm losing... control...!!"
Gohan reached this form once, while losing against Majin Broly. He was able to defeat Broly (Broly even if he was resurrected would be an ant to even Pan at this stage) following his transformation, but ended up losing control and began attacking the city. Goku was ultimately able to defeat him with the use of Super Saiyan 9 after a close fight, rending Gohan unconscious and powered-down. The form was never used after that, due to being too much of a potential hazard-at least, for a while. But then, Goku and Vegeta figured out how to reach this form and not lose their minds, and when they reach this form, they figure out how to power down as well. They then taught the others and it led to Gohan not being the only Saiyan who can reach this form.
Super Saiyan 8 has waist-length, spikey, brown hair, black fur, and a red aura with red sparks and red electricity. The symbol of the form's control over it's user is present as a red S on the user's head. This is similiar to Majins only they have fancy Ms on their heads.
Usage & Power
Super Saiyan 8 multiplies the power of the Super Saiyan 7 by 40. This is exactly why the form is so monstrous and exactly why defeating it as a Super Saiyan 7 is simply not an option. In this form, Gohan gains a signature move: the Dark Masenko.
Super Saiyan 8's name is sort of betraying. This is due to the fact of being one of two successor transformations to Super Saiyan 7. The other is Super Saiyan 9. However, Super Saiyan 9 is only 2x as powerful as Super Saiyan 8. This makes it a little hard for Goku as a Super Saiyan 9 to defeat Gohan as a Super Saiyan 8.