When the mutants left Earth to avoid persecution, just prior the Martian invasion in the 20th Century, the exodus eventually led them to a habitable planet they named Haven. Over time the number of mutants dwindled as more and more of the births resulted in baseline humans, until the nine remaining mutants became an elite ruling over the humans with an iron hand, with Rancor the undisputed leader of the mutants.
A resistance developed among the humans, and when the mutants eventually tracked them down, they discovered that one of their own, Bear, was collaborating with them. They killed Bear and the other resistance members they found at the meeting. They were soon contacted by the Guardians of the Galaxy, who had just located Haven, while searching for the long-lost mutants. Rancor and her lieutenants were suspicious of the newcomers but met with a few of them. When they discovered that other Guardians were meeting with the underground resistance, Rancor attacked them, ordering the mutants to put down the revolting humans.
They battled the Guardians, losing two of their number, Rhodney and Blaster, and having to face the threat of a new Phoenix host being empowered. Realizing there was little chance of victory, Rancor detonated bombs to destroy the planet, determined that if she couldn't rule haven, no one could, and led the mutants to safety, while the Guardians helped the humans escape to safety on Main Frame's world.
The mutants followed Rancor as she began a quest to discover what had happened to her ancestor, Logan. In a museum near the former Shi'Ar Empire, they discovered one of his broken claws on display, which Rancor took.
The search led them then to Earth, where they again came into conflict with the Guardians, where Rancor battled the new Guardian, Talon, but they were kidnapped by Doctor Doom, who forced their obedience, causing violent resentment in Rancor, who took some time to realize she couldn't easily kill him.
They worked for him for a while, again coming into conflict with the Guardians, while Rancor used surprise to attack Doom. The battle was close at times, but ultimately the Guardians won, capturing the mutants. While it initially appeared that Rancor had won, Doom soon rose, revealing that his body had long gone, his brain kept in a cyborg body which included the adamantium skeleton he'd taken from Logan, encasing Victor von Doom's brain. Rancor survives the fight due to the intervention of the Guardian, Yellowjacket, and escapes Guardian custody, stealing their med-shuttle, and freeing her Lieutenants.
Learning that the Inhumans had recently been abandoned by their dictator, Loki, they took advantage of it by enslaving them, and loaded them onto a spaceship they forced them to build, setting off in search of a new home-world. They found one, naming it Haven II, where they set the Inhumans to work building for them.
While the Guardians were off their ship, Rancor dispatched Sidestep, who, aided by Mindscan, teleported onto it, and set it up to gas the Guardians unconscious when they returned to it. The trap worked, and the unconscious Guardians were transported to Haven II, where they were set against individual mutants. Shaddo was pitted against Nikki, who bested the mutant, and would have killed her had Vance not interrupted her. The Guardians were eventually teleported away by Talon.
Shaddo was created by Jim Valentino in 1991 and first appeared in Guardians Of The Galaxy # 8.