Abraxas is a villainous cosmic entity in the Marvel universe. He was released when Galactus seemingly died in the Galactus mini-series, whose main opponents were the Fantastic Four as well as a number of cosmic-heroes/beings from across the Marvel universe - although due to the vast nature of his power it could be argued Abraxas has the honor of being the opponent of practically every being in Marvel - hero and villain alike.
Abraxas is one of the many cosmic-entities of the Marvel Universe, his origins dating back at least as far as the Big Bang - if not further - he is said to be imprisoned within Eternity itself and can only break free of this prison when Galactus dies, upon release Abraxas is a relentless destroyer of universes who seems to embody the end of all things.
Reign Of Terror
He sought to destroy the Marvel universe and all its related alternate-realities, sometimes referred to as the Multiverse - he sought to achieve this by using the Ultimate Nullifier, a machine that had the potential to erase all of reality on a whim if welded by one with enough power.
Unfortunately for Abraxas, the only person who knew the location of the Ultimate Nullifier was a certain Human-Torch, brother of the Invisible-Woman and member of the Fantastic Four.
In order to lure the heroes to himself Abraxas murdered an alternate-reality Galactus and sent his skull to the main Marvel Earth - this caused a rift in the space-time continuum that allowed beings from other realities to enter the mainstream-universe - amongst them an alternate-version of the Herald known as Nova.
The Fantastic Four try to seek an answer via going to the wise alien known as the Watcher only to find that Abraxas has made him forgot who he is, then puts the Watcher into a coma.
Reed Richard proceeds to gather a selection of superheroes to fight minions Abraxas has placed on the moon (which is also the abode of the Watcher), during this battle the Silver Surfer is seemingly killed by a strange mist - Abraxas' minions seek out the Human Torch (and more specifically the location of the Ultimate Nullifier) - however, it is soon revealed the location is not within the mind of merely the mainstream Human Torch but rather shared between multiple versions of the hero across space/time.
The Fantastic Four (after some cosmic-traveling) manage to locate the Ultimate Nullifier only for the alternate Nova to turn traitor and reveal she was helping Abraxas all along - Abraxas then proceeds to destroy the protector of Eternity, Roma and is given the Ultimate Nullifier by Nova.
At this point a massive battle is waged in which the Fantastic Four are powerless against the might of Abraxas until Franklin Richards and Valeria von Doom combine their powers to resurrect Galactus from - causing the two of them to pretty much lose their powers in the process.
Galactus, having been returned to life, then take the Ultimate Nullifier from Abraxas but is distracted by Nova - causing the Ultimate Nullifier to fall into the hands of Mister Fantastic, who proceeds to reshape the multiverse in order to stop Abraxas - in essence causing all the death and destruction caused by Abraxas to have never occured.
Powers and Abilities
Abraxes is one of the most powerful of all Marvel's villains - capable of immense feats of cosmic might, however in order to achieve the massive goal of destroying the entire Multiverse it seems Abraxas needed some extra help in the form of the Ultimate Nullifier, the extent of Abraxas' power is so great that Reed Richards was forced to destroy and remake the entire Multiverse in order to defeat him - a feat worthy of respect even in a world of superheroes and villains.
Possibility of Return
Abraxas is said to be a part of Eternity, the destructive side of creation - going by this definition Reed's actions have not truly erased Abraxas but rather caused him to become imprisoned once more - awaiting the day that Galactus dies once more so that he may once again begin the end of all things.