While the Incredible Hulk may be one of the strongest characters in Marvel, his son Skaar may be even deadlier than his father ever was. Though the Hulk may seem like the last person to raise a family, he has more than a few Hulk children of his own, one of them is Skaar. Skaar was conceived when Hulk arrived on the planet Sakaar, a deadly planet ruled by a dictator, the Red King.
Though Hulk was initially enslaved to fight in gladiator matches, he slowly became a symbol of freedom, and eventually overcame the Red King and took his place as ruler of Sakaar. Hulk was not alone in this venture as he had several allies alongside him, one of them being Caiera the Oldstrong, an alien with the "Old Power" (an ability that allowed her to control the planet itself). Though there was some hostility between the two at first, Hulk and Caiera fell in love and, after Hulk became king, he took Caiera as his wife. Hulk and Caiera ruled Sakaar fairly and justly, and soon after they were wed the two were expecting their first child. However, after the destruction of Hulk's kingdom and the death of his wife, his child was presumed dead... but it wouldn't be long before the Son of Hulk appeared to claim his birthright.
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As Caiera was dying, she used some of her Old Power to protect her son, Skaar, from the explosion in a cocoon. After awhile, Skaar emerged from the cocoon, and with some guidance from his mother's spirit, he began to fight many of the creatures on Sakaar. Though Skaar was initially an infant, within a few months, he grew to the size of a small child, and eventually he met a shadow priest called Old Sam, of the same species as Caiera, who would raise the child to an extent. Old Sam considered Skaar the potential savior of Sakaar, being able to liberate the planet from Axeman Bone, a ruthless tyrant who enslaved many people on Sakaar, and killed those who defied him. Though Skaar was indeed powerful like his father, spending most of his time in a "Hulk-like" form, he still lacked the Old Power of his mother. Old Sam knew of a technological means to grant Skaar the Old Power, but it had fallen into the hands of the Red King, who had cheated death at the hands of the Hulk. Though his allies were reluctant, Skaar accepted the Old Power from the Red King, and Skaar's true powers were unleashed.
Once Skaar achieved the Old Power, he was able to sense all life on his home planet. He was also privy to the entire planet's history, and all the violence and death that his people had endured. Overwhelmed by this, Skaar had decided to use his Old Power to wipe the planet clean of all killers, and began to attack Axeman Bone's forces across the planet. Skaar, without moving, was able to precisely target people using the planet itself as a means to find and attack soldiers by using the planets crust and magma to do so. At one point, Skaar threatened to murder Bone's daughter, an innocent girl who was halfway across the planet, as a way to get him to surrender. Before Skaar could destroy the entire planet, the Silver Surfer soon arrived, as the Old Power found on Sakaar would be enough to satisfy his master, Galactus's, hunger for centuries. The Surfer stripped away the Old Power from Skaar, and eventually Galactus devoured the planet Sakaar, with only a small number of survivors able to escape. Though Sakaar was destroyed, Skaar landed on Earth where he found that he could still channel the Old Power through the Earth itself, making him just as lethal on Earth as he was on Sakaar.
While on Earth, Skaar used the Old Power to defeat many powerful foes like the Juggernaut, as well as a demonically possessed Doctor Strange. Similar to Sakaar, Skaar can use the Old Power to not only weaponize a planet, but he can also fix a planet, moving the earth's plates and lava to do so. This was demonstrated prominently when, during a battle against his father, Skaar prevented a Nuclear Containment Building from exploding using lava within the earth to seal the building, and shifting the earth so that some of the destruction caused during the fight was fixed. Drawing on the Old Power also grants Skaar immense strength that allows him to fight on par with the Hulk, and can allow him to transform into stone as a defensive measure. With this combination of abilities, on top of the fact that Skaar can use this power at any time (even if he isn't "Hulked Out"), Skaar may just be the strongest there is.
The Old Power is an incredibly powerful energy source, that was in fact created to rival the power cosmic that the Silver Surfer possesses. While it isn't as strong, it does explain how Skaar can enhance his strength, and utilize entire planets as weapons in such an effortless manner. Though Skaar is no longer a Hulk, as the Green Goliath himself (in his "Doc Green) persona transformed many "Hulked-Out-Heroes" back to their normal states, Skaar can still utilize his Old Power whenever he sees fit.
Skaar and the Old Power haven't been seen in awhile, but with the reemergence of the "Green Scar" persona from Planet Hulk, and the return of several Gamma Powered heroes over the years, it stands to reason that Skaar could return in some way. Should Skaar return, with the Old Power backing him, even The Hulk should be worried, as strong as he is, he can't weaponize entire worlds like his son can.