Some combat encounters begin with a surprise round, which occurs if any creatures are caught completely off guard at the start of battle. If even one creature is surprised, a surprise round occurs, and all creatures that aren’t surprised act in initiative order during that round. Surprised creatures can’t act at all during the surprise round.
Special Rules Two special rules apply to the surprise round.
Limited Action: If a creature is not surprised, it can take only one of the following actions on its turn during the surprise round: a standard action, a move action, or a minor action. The creature can also take free actions, but it cannot spend an action point. During the surprise round (but not on its turn), the creature can take an immediate action, as well as opportunity actions. See “Action Types” for definitions of these terms.
After every creature that is not surprised has acted, the surprise round ends, and creatures can act normally in subsequent rounds.
Surprised: If a creature is surprised, it can’t take any actions, not even free actions, during the surprise round. The creature also grants combat advantage. As soon as the surprise round ends, the creature is no longer surprised.
Surprised is one of many conditions that can be applied to a creature. See “Conditions”.
Published in Player's Handbook, page(s) 267, Rules Compendium, page(s) 191.