Try Robert Jordan's Wheel of Time saga. It's eleven books so far - the twelfth and final one is due out in 2009 - the first of which is The Eye Of The World. It is a little slow to start with, but things will gradually build up. It has everything you want; the setting is an unnamed world (called Randland among fans, after the protagonist) that lives in fear of the Dragon, the reincarnation of the greatest warrior who ever lived. Two thousand years previously, mankind rallied against Shai'tan - the world's resident Dark One - to seal him back in his prison. But his final act was to taint the male half of magic, corrupting its users which led to the destruction of the world. The Dragon is prophecised to return and deliver mankind from evil ... and destroy the world again. Standing between the Dragon Reborn and the Dark One are the thirteen Forsaken who were sealed with him, the Dark One's most devoted servants, hidden in every corner of the world.
Unfortunately, the last few books have been a little lacklustre, aside from a few moments (virtually nothing happens in one book, which is only used to serve as bridge the previous book an the next one), but it does have everything you want. Mythological creatures come in the shape of the Dark One's servants, beasts tamed by the mysterious Seanchan (you won't meet them until book two) and a few other races scattered around Randland. Magic isn't divided into white and black schools the way it would be in a video game, but into male and female halves with a third source reserved for the Forsaken. A lot of the story revolves around the protagonist's efforts to control the madness that will come with use of magic. It's not child-oriented, though the first 50-100 pages are reminiscent of Lord Of The Rings (the author said he did it deliberaty so fantasy fans would be in familiar territory), though I'd hardly consider LoTR as being intended for children. It is a little bit predictable at times - it's obvious the protagonist is the Dragon Reborn, for example - and doesn't really have a "good twist" a la the Final Fantasy games (bear in mind that we're not finished yet as one book is still to come out), but a twist is simply a literary device and not necessary to make a book good or not.
The series can get a little heavy at times and, particularly in the last few books, there's been a little bit of useless to-and-froing, but they're still worth a read.
