I liked the story, but I have a few notes:
* Sounds like an RPG (this wasn't mentioned in the text).
* The Archer's story feels lacking, compared to the others. I'd rethink it.
* The final epic battle ends with all 4 classes - why? It'd be nice for you, as a developer to see all classes fighting together, but the player, who has chosen 1 class to play with - doesn't know anything about the other 3 characters. He hasn't seen their story, hasn't played them, and has no connection to them. Unless I misunderstood the gameplay...
* After that final epic battle - the player goes to the king and rescues him, thus saving the kingdom. Sounds kinda anti-climactic. Why doesn't get to kill the villain? Also - if all magical items cause their users to go crazy - wouldn't people know that, and know to avoid magic items?
* Why did the king attack the ship? There should be a reason for it. And don't say "cause he's evil". Evil doers have reason. Logical reasons. A drug dealer will kill a cop to avoid going to prison. That's a perfectly good reason. So - why attack a simple fishing boat?
* Like shifty said - it's an idea. The premise of a game. Now - what are you going to do with it? How will you make it unique, special, interesting or fun?
Basically - it's a good start, but you still have a long way to go. Keep it up!