Ultra Rage is a specific type of rage boost first used by Seventy-Five. This is a transformation/powerup to the current form, and it makes the user 5x stronger. Xamo and all of his creations have it, as well as Viaton and possibly Shadora.
Ultra Rage causes the user to glow red, their body to become under a lot of pressure, and their aura to change. Skilled users can activate this at any point, though Seventy-Five's first usage of it took a while and caused him great stress.
The user's power multiplies by 5, but it leaves their body overwhelmed, though not as much as Kaioken. It can also be used for defense, as it boosts all stats.
Omega Seventy-Five accessed this after snapping, but it seems that his God form also has the ability, the latter being able to obtain it at will, and carry it into his next forms. Before using this powerup on his God form, he used it on his Gigapad form for a very brief amount of time after getting angry at Soul Weegee.
Vileon possesses the ability since he's an Xamo creation and only used it once against Star-X and attempted to kill him in one hit. He quickly ran out of power after using it. He attempts to use it once again against the old heroes in Season 4, but for once they attacked before he could complete it.
Rotu-4 possesses the ability since he's an Xamo creation and used it many times against the heroes. He exercises far better control over it than most users, even being able to block Viaton's absorption ability with it. Since the present Rotu-4 is currently alive and allied with the heroes, there's a chance he could perfect it.
Viaton absorbed Rotu-4 from the past and gained the ability. He immediately used it, but it overwhelmed him and he couldn't control its power. As such, he went over the "Deity Level Limit" and got sent to Universe 6.
Xamo used a perfected version of Ultra Rage against Universia and the heroes after time was changed. It's presumed that Xamo knew this in the original time as well, but he never used it then. Xamo can sustain the form for a lot longer than usual, as it doesn't use up much stamina, and the glow is less intense.
Much like her brother Xuma perfected Ultra Rage. Having the same properties as mentioned above. She can also stack her Beyond Limits "form" on it for even more power!
Shadora's ability to learn/copy techniques would lead to her learning how to use Ultra Rage thanks to her observations of Xamo, Seventy-Five, and Vileon. While not perfecting it she is still able to stack the kaioken onto it for greater power.
Hecon was seen in a cryotube when Xamo released Rotu-4. He can presumably use Ultra Rage due to being an Xamo creation. He never used it.
Squinian was seen in a cryotube when Xamo released Rotu-4. He can presumably use Ultra Rage due to being an Xamo creation. He never used it.
Zamon was seen in a cryotube when Xamo released Rotu-4. He doesn't use this technique in Season 3 as it would of put him beyond the Deity limit at the time, or he simply didn't know it yet. However once escaping the Mega Void in Seasnon 4 he does use it to harm Jupitais
Being a direct creation of Xamo after Seventy-Five Neitoru is shown to be able to use Ultra Rage to a perfect degree, able to set it at percentages like 33%. Howver he has only done it in the split timeline Universia caused, the current Neitoru has not used Ultra Rage yet.