The Death Gate Cycle

Ages ago, sorcerers of unmatched power sundered a world into four realms - sky, stone, fire and water - then vanished. Over time, magicians learned to work spells only in their own realms and forgot the others.
Now only the few who have survived the Labyrinth and crossed the Death Gate know of the presence of all four realms - and even they have yet to unravel the mysteries of their severed world.