The Newbies were a race of aliens that appear in the Doom novels, making their first and only appearance in "Endgame".
Little is known about the Newbies, but they have been described as being nomadic, travelling from world to world and enslaving or destroying any life forms they considered inferior. Their main characteristic was their ability to evolve at an extremely rapid pace, essentially becoming a brand new species within a short time span. This ability also gave them the capacity to revive destroyed tissue and organs should they suffer mortal wounds. They were also a hive collective, being able to communicate and share thoughts with each other.
The Newbies possessed a sort of superiority complex, resulting in their desire to gain dominion over lesser beings. This led a sect of them to the home world of the Freds, and the ensuing battles wiped the Freds off the map. When Flynn and Arlene make their way to the Fred home world, they first hear of the "Newbies" from a revived Fred corpse (the Fred was the one who called them "Newbies", the best name they could get from his incoherent, insane ramblings) and eventually find their own Newbie, perfectly crushed under the Fred ship.
Upon reviving the Newbie, they gathered some rather chilling information about them and decided to take a visit to their own home world. However, this particular Newbie had been dead for so long that it had become frightened of what its brethren had become, scared enough to try and turn the ship around. However, its pleadings fell on deaf ears and it was soundly killed.
They arrived on their world, dubbed "Skinwalker". Flynn and Arlene found no traces of the Newbies anywhere, but they did come across humans. After a series of events, it was discovered that, over the years, the Newbies had evolved to the point where they were no longer physical but were, now, molecule-sized beings, a set of triple-helix DNA strands, which had the capacity to infect any creatures they find themselves inhabiting. In this state, they had the ability to control their hosts, albeit briefly. These Newbies now called themselves the "Rescucitators".
What Newbies still remained on Skinwalker in this state infected a portion of the human soldiers, though they were unable to infect Flynn and Arlene. In doing so, they are able to placate much of their hosts' thanatophobia and use their improved physical traits to their advantage. However, for reasons unexplained, faith is able to destroy them; their deaths caused any human who infected them to be free of their hold. It was faith in Flynn saved a number of the soldiers, but not all. Those soldiers that didn't listen, now called the "res-men", were able to incapacitate the two marines, hold them captive, and use the vacant Fred ship to depart for another trip to the war torn Earth.
However, in the computer simulation that the duplicated souls of Flynn and Arlene were in, a "back door" into a res-man is found, and they found themselves facing an actual "Newbie soul". Flynn brought the soul back into the simulator, which ran at a speed MUCH faster than real time, and as a result, and through further remembrance on Flynn and Arlene's part (including remembering that the Newbies evolved to the point where they didn't care about Earth and the universe anymore), the soul's evolutionary speed became so fast that it was a different entity every few seconds until, eventually, it evolved completely out of existence. This course of action caused all the Newbies to do the same due to their hive collective nature.
When the real Flynn and Arlene returned to Earth, they found no trace of the ship that was bringing the res-men to Earth. They had no idea on what had transpired, but returned to Earth and thought no more of them anyway.