Can a character be surprised mid-combat?

Question

Imagine the following scenario:

• Four adventurers are traveling from one city to another. They have 1 horse which is pulling a covered wagon.
• One adventurer is riding and controlling the horse, two are walking alongside, and the fourth is resting in the wagon (not sleeping).
• The party is ambushed by a group of monsters. Let's assume that all the PC's are surprised and only the monsters take their turn. However the monsters do not know that there is a fourth character in the wagon.
• The two characters who were walking begin fighting the monsters. The character on the horse stops it, dismounts and grabs his weapons. He does not begin fighting because he has already used all of his actions interacting with objects.
• Meanwhile, the character in the wagon hears fighting and decides that he should join in. He gathers his weapons etc and gets ready to fight.
• A few rounds later, the character jumps out of the wagon and attacks one of the monsters.

What happens? Does the attacking player get advantage? Is there some sort of surprise round? (I know there are no surprise rounds in 5e, but you get the point) Does nothing happen because the monster is already in combat and alert to all threats, including unexpected ones?

Answer 1

No, a character can't be surprised in the middle of combat.

Any character or monster that doesn’t notice a threat is surprised at the start of the encounter.

This is the only method given in the rules for a creature to be surprised. The monster already noticed a threat, so it can't be surprised, and besides, it's not the start of the encounter.

That said, it's certainly reasonable for the player in question to count as hidden while concealed in the wagon.

In combat, most creatures stay alert for signs of danger all around, so if you come out of hiding and approach a creature, it usually sees you. However, under certain circumstances, the Dungeon Master might allow you to stay hidden as you approach a creature that is distracted, allowing you to gain advantage on an attack before you are seen.

These sound like the right sort of circumstances for the character to stay hidden as they move to attack, since the monsters don't even know there is another attacker. As it says, this is up to the DM, but it's not much of a jump, particularly if the player who was in the wagon makes a Stealth check to approach sneakily.

So while you can't get the benefits of surprising an enemy mid-combat, you can probably at least get advantage on your first attack.