The Cancerverse is a twisted, organic mass of unimaginable size which bears resemblance to a cancerous tumor with suction-cup covered tentacles and circular "mouths" that are literally overflowing with razor-sharp teeth. Within this deathless corpse is a dark, abysmal alternate version of The Marvel Universe that pulsates with corrupt forms of life.
Much to the dismay of its inhabitants, Earth-616 became connected to The Cancerverse through The Fault, a gouged wound in time/space which formed a tunnel between the two worlds after a Terrigen Bomb detonated in between Kree and Shi'ar space at the end of War of Kings.
Not much is known about the actual history of this reality other than that it was consumed and taken over by Shuma-Gorath and it's foul brethren The Many-Angled Ones , who are better known to their followers as Kthl, Nyerlathortech and Yot-Soter. It does have its own Earth and like the rest of The Cancerverse it is twisted and corrupt. Those who are known to inhabit the "Cancerearth" are Lord Mar-Vell, his right hand Vaughn, The Hulk, The Revengers and The Defenders of The Realm.
Recently, the accursed leader of The Universal Church of Truth, Adam Magus, detonated dozens of converted church worlds at the base of The Fault, ripping it open and allowing The Cancerverse to ooze out into The Marvel Universe like a stream of rotten pus flowing from a lanced boil. This catastrophe has also formed a tangible bond between the rival universes, essentially turning them into extensions of one another.