One can purchase Warcraft III Battle Chest from Amazon for a PC or Mac for $19.99. There are several other places that one can purchase this game as well.

You do not 'unlock' anything in Warcraft III story related. They come naturally as you continue through the game.

However, I am sorry to inform you that if you do not own the expansion to Warcraft III: Rain of Chaos, you cannot play the Human campaign with Arthas found in Warcraft III: The Frozen Throne.

You can purchase The Frozen Throne at a local store in the popular 'Battle Chest' box, or order it online.

Go to your battle.net account management homepage.

Click on "Add Games"

Click "World Of Warcraft Battlechest"

Click "Serial code/Have serial key"

Enter the Serial code provided when you purchased the battle chest.

The sorceress unit in Warcraft III is a spellcaster that can be used to slow down enemies or polymorph them into something useless to take them out of the battle. She can also turn invisible.