Aftermath was a villain from Marvel UK - one of a number of "British Sleepers" created to defend Britain from potential nuclear attack he went rogue and fought against some notable Marvel heroes.
During the events of the Gulf War in the early 1990s, the third in a series of British Sleepers was created using the Freestate Technology invented/discovered by Herbert Isaacs. This Sleeper was created to defend England in the event that the conflict escalated to nuclear level. The Sleeper, composed of solid light, would be unharmed by nuclear attacks, and could continue to fight for England if this occurred.
Following a series of shifts in the government, after the conclusion of that war, the third Sleeper was stored in inert form in an unused aviary.
Oonagh Mullarkey awakened the Sleepers, which began to activate in succession. Believing they were attacking the enemies of England who had been victorious in their past wars and overtaken the country, the Sleepers rampaged and attacked everyone they encountered. The first two Sleepers were dispatched by a group of heroes.
The Silver Surfer sensed the power contained in the third Sleeper and attempted to strike preemptively, obliterating it before it could come to life. However, the third Sleeper absorbed the Silver Surfer's energies and evolved into the powerful being known as Aftermath. Like the previous Sleepers, Aftermath believed that England had been conquered by its enemies, and set out to destroy everything in England, so that the enemies wouldn't have a single stick or stone of which to make use. Aftermath soon evolved beyond its programming, and developed the new goal of conquest of Earth. Neither the heroes opposing it, nor Tommy, the first Sleeper (who had shed its programming as well, but was now helping the heroes), could stand up to Aftermath. Oonagh Mullarkey attempted to take control of Aftermath and use it for its own purposes, but is easily threw off her attempts and decided to locate and destroy her.
Ultimately the Doorman revealed his true nature as the creator of the Sleepers, and gave the heroes instructions on how to overcome Aftermath. Plasmer reformed herself into an ultra-dense mass, which the Doorman used to contain the power of Aftermath, so that it could not release any of its energies. The Silver Surfer then poured his full power into Aftermath, causing it to overload and blow up.