Surprise

Surprise occurs when one side in combat catches the other off guard. There is no "flat-footed" condition in 5e — surprise is the only mechanic for ambush situations.

Determining surprise: The DM compares the Dexterity (Stealth) checks of the hiding side against the passive Perception of each creature on the other side. Any creature that doesn't notice a threat is surprised.

Effects of being surprised:

• A surprised creature can't move or take actions on its first turn of combat.
• A surprised creature can't take reactions until its first turn ends.
• Once that turn ends (even if the creature did nothing), the creature is no longer surprised and can react normally.

Important clarifications:

• Surprise is not a condition — it's a state that lasts only until the end of the surprised creature's first turn.
• Initiative is rolled normally for all creatures, including surprised ones. A surprised creature still takes its turn in initiative order; it just can't do anything on that turn.
• Not all members of a group need to be surprised. Some might be alert while others are caught off guard.
• The Assassin Rogue's Assassinate feature specifically interacts with surprise, granting advantage and automatic critical hits against surprised creatures.
• The Alert feat prevents a creature from being surprised.