Entering Combat

If a player or other creature takes an action that initiates a combat (such as casting a spell, making an attack, or using a feature), that action will already be spent on this creature’s first turn of combat. 

• As an example: A player opens up a combat by casting the Hold Person spell, while another player immediately makes a melee attack against the creature being targeted by the spell. At the beginning of combat, the player who cast the spell will be able to take their turn as normal, but will not have their action on this first turn of combat. The same is true for the player making a melee attack, but if they have the Extra Attack feature, they will still be able to make one more attack on their turn (or more, if they can make more attacks than two during their attack action).

Surprise

A band of adventurers sneaks up on a bandit camp, springing from the trees to attack them. A gelatinous cube glides down a dungeon passage, unnoticed by the adventurers until the cube engulfs one of them. In these situations, a creature has surprised another one side of the battle gains surprise over the other.

The DM determines who might be surprised. If no creatures involved in the combat try to be stealthy, they automatically notice one another. Otherwise, the DM compares the Dexterity (Stealth) checks of anyone hiding with the passive Wisdom (Perception) score of creatures on the opposing side. Any character or monster that doesn’t notice a threat is surprised at the start of the encounter by the threat.

The surprised condition has the following effects:

• Surprised creatures cannot take a reaction against creatures that surprised them until their turn ends.
• The first attack made against a surprised creature by each creature who surprised it has advantage.
• If a creature is surprised by all creatures on the opposing side of an encounter, they make their initiative check at disadvantage.

If you’re surprised, you can’t move or take an action on your first turn of the combat, and you can’t take a reaction until that turn ends. A member of a group can be surprised even if the other members aren’t.