Very little is known about Abyss; he was first seen on the island of Genosha, where he was the bodyguard of the Genoshan researcher Renee Majcomb, who was investigating the Legacy virus.
Later, Abyss appeared in Berlin, fighting the mutant Fever Pitch who was trying to kidnap him. Abyss and Fever Pitch were both captured by the X-Corps, a mutant militia group founded by Banshee. Nils was held in a containment prison; from which he witnessed the death of Sunpyre, who was killed by Mystique. He then escaped from his confinement thanks to Banshee and to protect him Nils sucked Mystique into the void within his body.
The next thing we know about Abyss is that he joined forces with Nightcrawler, his half brother, and the X-Men to stop Azazel from invading the Earth with his other-dimensional army.
M-Day & X-Cell
Abyss is a survivor of the Legacy Virus but was de-powered after M-Day. He recently joined a group of de-powered mutants known as the X-Cell. Abyss temporarily regained his mutation thanks to Quicksilver. After seeing Elijah Cross die due to the side-effects of Quicksilver's unstable re-powering talents, Abyss took Fatale and Reaper and left for the Brimstone Dimension as he thought it may save them from exploding and losing their powers again. Abyss's fate is yet to be revealed.