What are the Whitewalkers in Game of Thrones?

I recently got thinking about the Whitewalkers from Game of Thrones again for some reason  and I wanted to consider what they are and why they are now appearing. Of course most of this is speculation but we know a few things that might hint at the bigger picture.
The first thing we know is that they are in some way necromancers. For those of you not well versed in fantasy a necromancer is a wizard who controls the dead. The most common way they do this is raising bodies from the grave and uses them as soldiers. But there is a subcategory of necromancer that seems to fit the whitewalkers better and that is a lich. A lich is a powerful necromancer who effectively kills himself and rises as an undead creature to gain immortality and power. This explains why the whitewalkers look dead themselves. The cold aspect of them is sperate from this but cold, darkness and death are often connected so it’s not a stretch.
The other thing that I think we can deduce is that they are enemies of the valyarians or the dragons. This is because the two things we know can hurt them are both connected to Valyria and dragons. Dragonglass and Valyrian steel. This seems too big of a coincidence to be simple chance. But beyond that there is a third reason to believe that they have some connection valyria which is the timing of their return.
According to everything we know the Whitewalkers have been dormant or gone for thousands of years. That suggests they have returned at this time for a reason. The biggest change we know of that has happened is the destruction of house Targaryan. This has happened a while ago in the books, but if you consider the way that things work in this world it makes a bit more sense.
In the game of thrones world seasons are far longer. Summer in the game of thrones can last for seven years and winter potentially even longer. So assuming that the Whitewalkers are strongest in the winter then it might take them a bit of time to gain their strength. If Roberts Rebellion happened about 16 to 14 years before the beginning of the books. Perhaps a bit longer in the TV series since the children tend to be a bit older. But since winter is on its way we can assume there was probably only one winter between the rebellion and the coming winter. Assuming the whitewalkers used the first winter to gather power this would be the first real chance they had to attack since the rebellion and the destruction of the last people they were really afraid of. This timing is why I think it might actually be the Targaryans rather than the dragons, though both are connected. 
This is where I get into a bit wilder theory. People generally think of the whitewalkers as a different race, or a race at all. I suspect it may be something a bit different. First because we seem more than one of the riders people tend to assume they are the same, but there are some suggestions that there is a head whitewalker and that he is the only one who can create other whitewalkers. To me this seems to fit the personality of the whitewalkers more because while they act differently than the zombies they really don’t show all that much more independent thought.
So what does this lead me to. That perhaps the whitewalkers really are a singular enemy. A lich with the power to raise armies of the dead. Beyond this I suspect he might be from Old Valyria. I think it’s even possible that this character might be connected in some way to the Doom of Valyria, though it seems likely not entirely directly since the destruction of Valyria seems to be different than the way the whitewalkers work. That said, we do know that much of the magic of Old Valyria was lost which would make those who still knew that magic far more powerful in comparison afterward. It also leads to a potential point at which we could compare the current game of thrones with houses  fighting for the throne of Westeros to the battle between the houses of Old Valarya which would also be a sort of game of thrones but on a far different scale and giving the last member or members of house Targaryan a true enemy to fight. Also who doesn’t want to see dragons fighting the whitewalkers at some point. 

I recently got thinking about the Whitewalkers from Game of Thrones again for some reason  and I wanted to consider what they are and why they are now appearing. Of course most of this is speculation but we know a few things that might hint at the bigger picture.
The first thing we know is that they are in some way necromancers. For those of you not well versed in fantasy a necromancer is a wizard who controls the dead. The most common way they do this is raising bodies from the grave and uses them as soldiers. But there is a subcategory of necromancer that seems to fit the whitewalkers better and that is a lich. A lich is a powerful necromancer who effectively kills himself and rises as an undead creature to gain immortality and power. This explains why the whitewalkers look dead themselves. The cold aspect of them is sperate from this but cold, darkness and death are often connected so it’s not a stretch.
The other thing that I think we can deduce is that they are enemies of the valyarians or the dragons. This is because the two things we know can hurt them are both connected to Valyria and dragons. Dragonglass and Valyrian steel. This seems too big of a coincidence to be simple chance. But beyond that there is a third reason to believe that they have some connection valyria which is the timing of their return.
According to everything we know the Whitewalkers have been dormant or gone for thousands of years. That suggests they have returned at this time for a reason. The biggest change we know of that has happened is the destruction of house Targaryan. This has happened a while ago in the books, but if you consider the way that things work in this world it makes a bit more sense.
In the game of thrones world seasons are far longer. Summer in the game of thrones can last for seven years and winter potentially even longer. So assuming that the Whitewalkers are strongest in the winter then it might take them a bit of time to gain their strength. If Roberts Rebellion happened about 16 to 14 years before the beginning of the books. Perhaps a bit longer in the TV series since the children tend to be a bit older. But since winter is on its way we can assume there was probably only one winter between the rebellion and the coming winter. Assuming the whitewalkers used the first winter to gather power this would be the first real chance they had to attack since the rebellion and the destruction of the last people they were really afraid of. This timing is why I think it might actually be the Targaryans rather than the dragons, though both are connected. 
This is where I get into a bit wilder theory. People generally think of the whitewalkers as a different race, or a race at all. I suspect it may be something a bit different. First because we seem more than one of the riders people tend to assume they are the same, but there are some suggestions that there is a head whitewalker and that he is the only one who can create other whitewalkers. To me this seems to fit the personality of the whitewalkers more because while they act differently than the zombies they really don’t show all that much more independent thought. 
So what does this lead me to. That perhaps the whitewalkers really are a singular enemy. A lich with the power to raise armies of the dead. Beyond this I suspect he might be from Old Valyria. I think it’s even possible that this character might be connected in some way to the Doom of Valyria, though it seems likely not entirely directly since the destruction of Valyria seems to be different than the way the whitewalkers work. That said, we do know that much of the magic of Old Valyria was lost which would make those who still knew that magic far more powerful in comparison afterward. It also leads to a potential point at which we could compare the current game of thrones with houses  fighting for the throne of Westeros to the battle between the houses of Old Valarya which would also be a sort of game of thrones but on a far different scale and giving the last member or members of house Targaryan a true enemy to fight. Also who doesn’t want to see dragons fighting the whitewalkers at some point.