When you encounter a party of adventurers, you will be told if an ambush occurs. If you ambush the adventurers, they won't be able to move in the first round. However, in this ambush round, you will not be able to cast spells or use elemental breath against them, as these moves are too involved to perform in a surprise maneuver. (You can, however, invoke items in an ambush round.)

Likewise, if the adventurers ambush you, they will only be able to fight hand-to-hand or invoke items in the free round of attacks they will get -- they won't be able to cast spells until the round after.

How frequently you'll be able to ambush adventurers depends largely on the monsters accompanying you at the time. Some monsters are better at ambushing than others. It also depends on the adventurers in question; the greater the agility and wisdom of the characters, the better they'll be at ambushing you and avoiding your own ambushes.

If you have a levitation spell currently in effect, this swings the odds of ambushes significantly in your favor.