What happens when you are hidden and a new enemy joins initiative?

Question

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.

This particular wording leads me to believe that if an enemy's first turn occurs after initial start of a combat encounter (ie. reinforcements), they could still suffer the Surprised condition as stated above from a single player if they aren't high enough in the initiative. Also, if one member can be surprised while others are not, could the same not be said of the character(s) initiating the surprise?

Story behind question below:

I was in a game where we were fighting a Hook Horror. Playing an Assassin Rogue, I was in Stealth and the passive perception (or active, for that matter) of the Hook Horror* couldn't meet my Stealth (Move Silently) check. As a result, I gained a single round before the rest of the party arrived to initiate combat with the Hook Horror in the Surprised condition. It called out, echoing through the tunnels and summoned more which were to arrive in a few turns. We dispatched it before reinforcements arrived, during which I rolled another Stealth (Move Silently). The next round came up and the reinforcements arrived. I was at the top of the initiative and got to go first and asked the DM if the new Hook Horror would be considered surprised as well, since it still couldn't beat my Move Silently stealth. Mulling it over for a few seconds, she decided that it would, in fact, be surprised as it had not yet been part of combat.

Obviously, this has already happened and won't change that it did, but for future reference, I'd like the input of some fellow rule nerds about how this should play out. The rules state that a Surprised creature cannot act on its first turn of the combat encounter, so the argument in question is, if a creature enters combat later than the original first round, can it still be surprised by one or more players who have opted for stealth?

*I know Hook Horrors have hearing-based blindsight and advantage on hearing-based Wis(Perception), but my stealth was a specifically a move silently check as we were in the Underdark and my character was a Drow with (rolled for) knowledge of such things, as well as other species/races having darkvision that would nullify a traditional Hide check.

Answer 1

Surprise happens before combat starts

According to the rules on the order of combat, is determined before initiative is even rolled:

COMBAT STEP-BY-STEP

1. Determine surprise. The DM determines whether anyone involved in the combat encounter is surprised.
2. Establish positions. The DM decides where all the characters and monsters are located. Given the adventurers' marching order or their stated positions in the room or other location, the DM figures out where the adversaries are--how far away and in what direction.
3. Roll initiative. Everyone involved in the combat encounter rolls initiative, determining the order of combatants' turns.
4. Take turns. Each participant in the battle takes a turn in initiative order.
5. Begin the next round. When everyone involved in the combat has had a turn, the round ends. Repeat step 4 until the fighting stops.

Even if you choose to determine surprise when the new enemies arrive, it is likely they are aware of at least one of your other party members, and so they would not be surprised.

However, if you are hiding then new enemies probably won't even know you are there

Logically when enemies enter an existing battle they aren't surprised, they are purposefully engaging with foes that they already know are there. When the enemies enter the field they will be able to attack your party members, but since you are hiding they won't realise you are there. This is very similar to surprise since you can attack them before they see you.

Answer 2

jgn has already succinctly shown that enemies cannot be surprised after combat.

However, there are game-impacting effects of being hidden when the new enemy arrives. If the PC is still hiding at the time of the new monster's arrival, you must resolve the following:

1. The value of the hiding PC's initial Dexterity (stealth) roll carries over. This means that if the monster's passive perception value is lower than the player's stealth roll, then the PC is hidden from the monster.
2. If the PC is hidden from the monster, then the PC's next attack against the creature will have advantage. This is as far as standard 5e rules go for granting bonuses for "surprising" an enemy mid-combat, but it does mean that a Rogue can use their Sneak Attack.

For more information on the benefits of hiding, you should read this fantastic answer.

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The following is advice for any disgruntled Assassin Rogues that come here seeking answers, which may not necessarily apply to OP:

The unfortunate reality of the Assassin Rogue is that, RAW, the vast majority of your offensive power is loaded into the first round of combat, not on whether or not you're hidden from enemies mid-combat.

If you want to maximize your offensive power, you'll have to do more as an Assassin than simply being the first to attack. If you want the best chance to land a critical strike against a surprised enemy, make sure that you're using your Rogue skills to scout the battlefield before combat starts, and make sure that you use stealth to your advantage at every moment pre-combat.

Once the first round of combat is over, then a large part of your character's features are no longer applicable, so make sure you do everything in your power to maximize your impact during that round.

Answer 3

Your DM should not have made the hook horror be surprised. A hook horror has the echolocation ability which powers it's blindsight. It doesn't matter how silently you were moving. Unless you were inside of an actual silence spell field it would have detected you since it would would hear the noise it produces bouncing off of you. That's how echolocation works.

Furthermore, surprise is assigned to a creature when it had no idea combat has been joined or is about to be joined. The new hook horror was called in by the original one. It should have been on it's guard.