"Mad" Jim Jaspers is a powerful supervillain from Marvel comics who was an enemy of Captain Britain and Excalibur as well as a few other heroes - despite his relative obscurity he is considered one of the most powerful mutants in existence, two versions of him existed and the most powerful one is the Earth-616 version (which is the mainstream Marvel Earth). His power can be summed up as so: the less powerful alternate-reality Jaspers was so dangerous his entire reality had to be erased to get rid of him: the more powerful mainstream Jaspers threat is so great that Merlyn has suggested he could become an Omniversal threat (which means he could theoretically become dangerous enough to destroy/take over entire universes): his current level of power after several defeats is unknown.
"Mad" Jim Jaspers (alternate)In the alternate reality of Earth-238 Jaspers led a criminal gang known as the "Crazy Gang" - inspired by Alice In Wonderland he played the part of the Mad Hatter
before he was a criminal however Jaspers was a British Politican to managed to outlaw superheroes - then proceeded to create a powerful being by the bame of The Fury to exterminate any remaining superhuman beings on the planet, within two years the world was almost barren of superhuman beings except Jaspers, who had programmed the Fury to not try and kill him. After Captain Britain arrived in this reality and foiled a bank robbery Jaspers reactivated the Fury - which proceeded to kill both Captain Britain and Jackdaw, however Captain Britain was revived by Merlyn and return to Earth-616 (mainstream Marvel). The Fury detected this and adapted itself so as to travel across realities. However when Jaspers unleashed the power of the Jaspers Warp Mandragon destroyed the entire Earth-238 reality out of fears of contamination, presumably this event killed off this particular version of Jaspers.
"Mad" Jim Jaspers (mainstream)
Upon returning to mainstream reality Captain Britain found that another Sir James Jaspers was leading an anti-superhero campaign, with the aid of Henry Peter Gyrich and Sebastian Shaw - members of the Hellfire Club (and strongly suggesting Jaspers is also a member of this organisation). Jaspers managed to win a landslide general election victory on his anti-superhero platform and became Prime Minister of Britain, events began to mirror those of Earth-238 and Jaspers unleashed the force of the Jaspers Warp upon London - causing much chaos and devastation. However (and somewhat ironically) Jaspers found himself under attack from the reality-hopping Fury (which realised he was not the same Jaspers he was programmed not to kill) and the two engage in a large-scale reality-warping battle in which Jaspers was ultimately killed by the Fury - who teleported him outside the universe and since Jaspers no longer had any reality to contol he was rendered powerless. However thanks to the events of M-Day and the Scarlet Witch's rewriting of reality "Mad" Jim Jaspers has been reborn - fused with the Fury as a single combined being.
Jaspers has vast superhuman powers derived from the concept of reality-warping, via this extremely potent but dangerous power Jaspers is capable of godlike power on a near-limitless level: to Jaspers near to nothing is impossible since he can literally rewrite reality into anything he wishes it to be, as long as their is a universe for him to manipulate Jaspers could be seen as unstoppable, although some beings of phenomenal power (such as the Fury) may be a match for him.The mainstream Jaspers is more powerful than his alternate one, yet even the weaker alternate Jaspers required the destruction of his entire universe to stop his madness - it could be possible that mainstream Jaspers is too vast for even that extreme act to be effective against him.
Although godlike in scale Jaspers does have a noticable weakness of sorts: use of his power causes severe deteration in his sanity - at his ultimate power-level Jaspers is completely insane and thus it could be said the old rule of ultimate power corrupting is true in the case of Jim Jaspers.
The Jaspers Warp
Another notable effect of Jaspers immense power is the use of a powerful but ultimately cataclysmic force known as the Jaspers Warp, by using this power Jaspers was able to tear apart entire realities - the effects of the Jaspers Warp also severely mutated individuals born during the period it was active (these unstable being were later refered to as "Warpies" and continued being born long after Jaspers defeat). Many areas of London were still experiencing detrimental effects on reality due to the power of this Warp even after the death of Jaspers himself - making it one of his most devastating qualities and a force sufficiently feared that Mandragon destroyed the entire Earth-238 reality out of a fear of becoming contaminated by its effects.