What is Napoleon's role on Animal Farm and what happens to him?

Napoleon’s role on Animal Farm is leader and dictator, and he eventually becomes indistinguishable from a human being. 

Old Major the pig is the one who comes up with the original vision for Animal Farm, but Napoleon is the one who acts on it.  When Old Major dies, Napoleon is the driving force for rebelling against the humans and organizing the animals into a human-free farm.  He is a tyrant and eventually no one can tell the difference between him and his minions and the people. 

From the beginning, the two pigs Napoleon and Snowball vie for power. 

Napoleon was a large, rather fierce−looking Berkshire boar, the only Berkshire on the farm, not much of a talker, but with a reputation for getting his own way. Snowball was a more vivacious pig than Napoleon, quicker in speech and more inventive, but was not considered to have the same depth of character. (Ch. 2) 

Napoleon begins by contradicting Snowball, quietly and then actively.  Snowball believes in a more literal and idealistic version of Old Major’s dream.  He wants the farm to build a windmill to electrify the farm and prefers for decisions to be made by committee.  Napoleon just wants power. 

After initially opposing Snowball, Napoleon drives him off.  He had secretly trained his own personal security force of guard dogs.  Despite decrying Snowball’s windmill designs, he announced that the windmill would be built.  It was a good way to keep everyone busy. 

The pigs gained more and more control over the farm.  It began with small things that separated the pigs from the other animals, such as the disappearing milk and apples.  Then the pigs began changing the commandments that they had instituted to support Old Major’s vision.  Squealer, Napoleon’s mouthpiece, explains away all of Napoleon’s decisions.  The other animals just accept them. 

The pigs had moved into the farmhouse, learned to walk on two legs, and even traded with humans for alcohol.  Napoleon gained further control over the farm with a campaign of terror.  Animals were afraid to speak against him because of the guard dogs, and he made an example out of some of the animals. 

The four pigs waited, trembling, with guilt written on every line of their countenances. Napoleon now called upon them to confess their crimes. They were the same four pigs as had protested when Napoleon abolished the Sunday Meetings. (Ch. 7) 

By the end of the book, Napoleon and the other pigs are playing cards with the humans and when the animals look in, they can’t tell the pigs from the people.  Napoleon went from being a respected and revered leader to a tyrant who cared for nothing more than gaining and keeping power.