Darkseekers are the result of a failed experiment to develop a cure for cancer. This miracle cure was distributed freely among the population. However, not long after its distribution, this so-called cure mutated into a rabies-like virus, killing much of the planet's population. Those who weren't killed by the virus mutated into savage, aggressive creatures that hungered for living flesh, whether it be human or animal. The virus could also mutate animals into Darkseekers.
Over the course of a few months, humanity was driven to the brink of extinction. A small cluster of survivors had established a colony in Alaska free of the infection, leaving almost no one left in the ravaged cities. However, one survivor - a scientist known as Robert Neville - remained in the desolate city of New York and had fortified his home to protect himself while he researched a cure for the virus. Turning his basement into a laboratory, he experimented on small infected animals such as rats and later moved on to testing on a human Darkseeker he managed to capture.
Little did Neville suspect that the Darkseekers were not as mindless as they seemed; the creatures had developed a pack or tribal social system and were aware that Neville was watching them, but they in turn were watching him as well. One night, a tribe of Darkseekers were able to breach the defences of Neville's house and they captured him. They tied him to a lamp post and left him there for a full day before coming out at sunset to kill him. Neville managed to escape with the aid of his pet dog Sam, but as they made their getaway, the Darkseekers released a pair of their own dogs which pursued them. Sam was bitten by one of the Darkseeker hounds and infected with the virus, forcing Neville to kill Sam before he turned completely.
Robert then went on a self-destructive rampage to kill himself and as many Darkseekers he could. Running several of the infected down with his vehicle before it was flipped over, his life was saved by the arrival of two other immune survivors; Anna and Ethan, both looking to make it to a survivors colony at a higher altitude where the virus couldn’t reach.
Later that night, it was discovered that the Darkseeker tribe from the previous night had followed them back to Robert’s house, and now an all out war had started. Overwhelming them in numbers, the creatures broke into Robert’s house, forcing the three into his basement where, just at the last second, Robert realized the cure was in the blood of his last test subject.
Getting a sample of it and giving it to Anna, he pushed the two into a secret hiding spot and readied himself to die. Pulling out a grenade, he pulled the pin and rushed the creatures just as the leader broke the glass separating the two. The grenade exploded, destroying the many monsters inside. The next morning, Anna and Ethan emerged and once again started their voyage, and soon ending it when delivering the cure to the colony.
The Darkseeker virus is known to infect humans, dogs and rats, though likely infects other mammalian creatures too. Its effects on other animal classes is unknown. Known infected species all suffer the same symptoms: complete loss of body hair, greying of the skin and greatly increased aggression towards uninfected creatures.
In the case of human Darkseekers, they seem to lose all powers of reason, becoming completely savage. However, they do still retain some degree of intelligence and even form some sort of crude society after such a long time since the initial outbreak. The Darkseekers are shown to behave in a somewhat tribal fashion; the group that Robert Neville had dealt with seemed to follow an 'alpha male', the largest and strongest among them.
As the name implies, Darkseekers are nocturnal creatures. They cannot survive in daylight as their mutation as afflicted them with a deadly intolerance to ultraviolet rays. Exposure to the sun's rays will slowly burn their skin, but they appear to die from some kind of shock long before their bodies are destroyed by the sunlight.
- Originally the Darkseekers were supposed to be intelligent and caring creatures such as found with the alternate ending of I Am Legend where the female is rescued by the Alpha Darkseeker. There are even hints of this in the movie as Neville got caught in a trap that was originally going to be passed off as Darkseekers' handiwork but this was evidently scrapped.