She was a villain whose soul was expelled from his body during a battle with Tecton, and is now a disembodied black cloud with hopes of returning to human form to take over the world.
In Lockdown, Revengeance snuck into Mighty Med Hospital through the ventilation system and took over Horace's body, as well as most of the hospital staff. Revengeance controlled the staff to turn on Mighty Med's positronic reactor to make himself human again. Revengeance gathered all nine Mighty Administrators, and set them in a specific formation to activate the access code for the reactor. Before Revengeance returned to his physical form, Alan, Oliver and Kaz used a vacuum to suck Revengeance up, and powered it with Surge's electric manipulation, thus defeating him.