Adam-X is theoretically one of Marvel's most dangerous mutants thats to a unique power that could bring down entire planets. But there are limits to what he can do, as he (and the rest of the X-Men) discovered when they came up against one of the most powerful incarnations of Juggernaut ever.
Fear Itself was the major Marvel crossover of 2010, centring around the global chaos introduced to the world by the Red Skull's daughter, Sin. Taking up a hammer of the lost Asgardian Cul -- otherwise known as the Serpent -- Sin was transformed into the powerful Skadi, and helped bring more hammers to Earth, forcibly corrupting and empowering powerhouses like the Juggernaut into new forms. For the Juggernaut, that meant becoming Kuurth, Breaker of Stone. At the same time, the X-Men had relocated across the United States to the island of Utopia off the coast of San Fransisco. Fighting against the human forces who wanted to use the Decimation of the mutant population to wipe the remaining mutants out, the X-Men found themselves increasingly protective of their island and San Francisco.
Juggernaut began his journey towards the heart of San Francisco in Uncanny X-Men #540-#543 by Kieron Gillen and Greg Land, throwing off almost every form of resistance thrown at him. To deal with the Juggernaut, Cyclops revealed the numerous strategies and plans he'd conceived of to defeat him, including deploying numerous different mutant heroes to try and slow his approach. But due to the inherent power of the Juggernaut -- coupled with his new mystical power boost -- there was little the X-Men could actually do to stop him. One of the mutants deployed to the battlefield was Adam-X. Secretly the genetic son of the former Shi'Ar Emperor D'Ken and Katherine Summers, Adam possesses increased speed and agility, as well as a potentially world-breaking mutation.
Adam is able to burn oxygenated blood and spread the effect through any target -- delivering immobilizing pain to a seemingly unlimited amount of people all at once. X-Men Legends recently showcased just how powerful Adam-X can be when fully engaged, with the young mutant using his powers to bring down an entire contingent of soldiers led by Erik the Red. At the time of Fear Itself, Adam had relocated to San Francisco with most of the world's remaining mutants. As such, he was one of the mutants called upon by Cyclops to engage Juggernaut -- with the theory being that bringing enough pain to Juggernaut would be enough to stop him. It's a plan that's worked before, including his most recent solo series when the Juggernaut was held at bay by a fusion of Toad and Swarm's powers.
However, the plan ended up being an outright failure. While the Juggernaut was unbothered by other assaults, Adam-X's actually made him more of a threat to the surrounding area: Although Adam-X is able to ignite Juggernaut's blood and it does seem to cause him severe pain, Juggernaut ignores it and continues making his way through San Francisco. As he continued on his path, the inflamed Juggernaut was able to actually spread the fire coursing through him -- igniting everything he touched. Ultimately, the only way to hold Juggernaut at bay is for Colossus to become the new Avatar of Cyttorak -- imbuing him with a new level of power and allowing him to force Juggernaut's retreat.
But Fear Itself is a good reminder that despite his impressive powers and his deadly potential, Adam-X does have his limits. Especially after he's been bosted in terms of power, there's almost nothing on Earth that can stop the Juggernaut -- and that means there are limits to what Adam-X can perform.