The Abyss shares many aspects with Set the Serpent God and supposedly pre-dates the Celestials suggesting he is an Elder God or a spawn of one. The Abyss spreads darkness across the Universe and taints all it touches. Much like Set, he was contained in a side dimension and seeks to free himself by opening a gate back to the real world by subverting the will of those who wear the Serpent Crown. Both the Archon and Luminal races consider The Abyss their sworn enemy and fight constant battles to contain it's evil forces.
The Abyss was created by Dan Abnett, Andy Lanning and Wellinton Alves and first appeared in Nova #8.
The Abyss' initial appearances in Nova were fairly straight forward and not much back-story was given. It eventually returned in the pages of Secret Avengers where writers Ed Brubaker and Rick Remender confusingly linked it to the Serpent Crown. Almost every aspect of the Abyss that has appeared in Secret Avengers is identical to that of Set the Serpent God, although Abyss also displays it's power as seen in Nova. It is entirely possible that while researching the Serpent Crown, someone at Marvel confused the Lemurians (a race linked to the Serpent Crown) with the Luminals (the race linked to The Abyss). What the Abyss' existence means for Set is unseen, but it appears that the Abyss has some what replaced Set.
Major Story Arcs
While on a mission to investigate Roxxon's oil operations on Mars, Nova discovered a Serpent Crown and through the power of Abyss was convinced to wear it. He then became possessed by Abyss and attacked his fellow Avengers until Steve Rogers used the Nova Force to defeat Nova and free him from Abyss' power.
Council of Masters
Max Fury sought to use the power of the Abyss to heighten his and the Shadow Council's power. After attempting to don the Serpent Crown to channel The Abyss and failing due to his robotic nature, Max Fury was defeated by the Avengers. When nobody was looking, Taskmaster took control of the Crown and became possessed by Abyss by wearing the crown himself.
Almost the entire population of Bagalia fell under the sway of Abyss, except for the Avengers' Ant-Man and Venom, and the mindless slaves of the Abyss sought to spread it's influence to the rest of the world. These plans were thwarted by Ant-Man and Venom when the Venom symbiote attempted to bond with Abyss and apparently relinquished his control over Bagalia.
Powers and Abilities
The Abyss has the power to turn people into mindless slaves, empower them with the ability to fire energy blasts.