The Fault is a rip in space and time over seven parsecs long, splitting the border between The Kree and Shi'ar Empires. It was created when the Inhumans T-Bomb exploded. At first, it expanded at a rapid rate, threatening to engulf all of reality, something that did not seem to bother an extra-dimensional tentacled creature that was trying to break it's way in to The Marvel Universe and metastasize its reality over theirs.
Adam Warlock was able to stop this expansion by overlapping time streams while The Guardians of The Galaxy and The Inhumans drove the creature back home. This left him weakened and resulted in him becoming the Magus, who was supposedly killed by Star-Lord. Some time after the Magus Incident, several different groups started to investigate the Fault, such as the Imperial Guard, the Inhumans and the Luminals.
The Luminals, who had gone on an unauthorized mission into The Fault, accidentally brought a strange xenomorph (that had been hiding inside of a Luminal named Massdriver's body) aboard Knowhere and Moondragon was forced to contain the creature inside of herself. From this, she was able to learn that much of what is inside The Fault is from other ruptured universes and that there is also a tunnel that links The Marvel Universe to the alternate reality which lays at the other end.
The reality on the other side, however, is a horrible, twisted universe, very unlike their own universe. As discovered by Quasar, who ventured into the Fault on behalf of Project PEGASUS, the Guardians of the Galaxy and the Nova Corps, it is a Cancerverse filled to bursting with corrupted life and ruled over by Lord Mar-Vell (whose existence currently remains unknown to all of Earth-616's heroes) and The Many-Angled Ones, a race of Dark Gods who are associated with The Great Old Ones and are know to consort with the likes of Chthulu and Yog-Sothoth from time to time.
Adam Magus recently detonated every world that The Universal Church of Truth had converted at the base of The Fault, causing it to tear open completely, allowing The Cancerverse to spill into The Marvel Universe like rotten puss flowing from a lanced boil, thus fusing the two realities together at that one singular point in time and space.
With the armies of The Cancerverse preoccupying all lesser forms of life, it would seem that the task of closing The Fault has fallen to the abstracts Oblivion and Eternity, who are taking up arms against The Many-Angled Ones and bringing the wrath of The Marvel Universe's most powerful cosmic beings down upon their heads.
A massive battle at the Fault began, led by Galactus and the other abstracts along with some of the greatest heroes of space.
The Fault was closed when Death appeared before the leader of the Cancerverse invasion, Lord Mar-Vell. She destroyed him and the entire Cancerverse.
Years later, the Fault re-opened, but was re-sealed by the Symbiote Hive before anything from the Cancerverse could emerge.