Rule 0: You can. But you can't - and you probably shouldn't.
Combat starts when the intent to attack starts (attack or do something else harmful). You should roll initiative, and, if the player character rolls higher than the sorcerer, they have the opportunity to do something. This is the whole point of the Alert feat and the initiative system: it doesn't ...
A surprised monster who rolls a 20+ on initiative can use lair actions because they are able to take actions, and their turn has passed
As you've quoted, the rules on lair actions state:
On initiative count 20 (losing all initiative ties), it can use one of its lair action options. It can’t do so while incapacitated or otherwise unable to take actions. ...
Surprise is very much up to the GM
How does surprise work?
The section on "The Order of Combat" states:
1. Determine surprise. The DM determines whether anyone involved in the combat encounter is surprised [...]
The section on "Surprise" states:
The DM determines who might be surprised. If neither side tries to be ...
Surprise is a specific term; it happens at the start of combat
The "Surprise" section of the PHB covers this mechanic:
The DM determines who might be surprised. If neither side tries to be stealthy, they automatically notice each other. Otherwise, the DM compares the Dexterity (Stealth) checks of anyone hiding with the passive Wisdom (Perception) ...
The rule you quoted states :
Any character or monster that doesn’t notice a threat is surprised at the start of the encounter.
And according to you the situation is :
The creature [...] goes straight to its source, entering the room and exposing its back to the Rogue who, unnoticed by the creature, prepares to attack it.
Therefore the creature is ...
Each character can be surprised individually
The rules for Surprise states:
The DM determines who might be surprised. If neither side tries to be stealthy, they automatically notice each other. Otherwise, the DM compares the Dexterity (Stealth) checks of anyone hiding with the passive Wisdom (Perception) score of each creature on the opposing side. Any ...
All of those would be fine and correct. As a DM you are the judge and it's up to you to decide which of these would apply to a certain situation.
You could pick just one and use it along the whole campaign or you can switch as you see fitting.
You could also clarify to your players why are you doing a group check in a given situation and why you would use ...
When the opposing party is no longer expecting a threat.
Narratively, your BBEG is fighting the party, and runs away, yelling
You'll pay for this one day!
Let's say the party, being cautious, tells you, the DM,
Ok, the Druid is downing a potion on the poor Barbarian, while the rest of us scout the remainder of his room for Mimics. Paladin is in front, I'...
The transition from not-Combat to Combat happens when the DM says it does. It doesn't have to happen on "turn boundaries".
Turn boundaries are an abstraction to keep combat simple.
Someone with the alert feat cannot be surprised (well, except by a plot twist or the like, but they still get to roll initiative). And entering combat doesn't happen ...
A surprised creature remains surprised until the end of their turn on the first round of combat
How often during combat can you be Surprised?
What happens when your group is victim of a surprise attack but you can't be surprised?
Readying an action for sneak attacks in surprise rounds
What's the correct way to determine turn order in this ...
"Look out for what?" Thwack!
When the players roll initiative, those who are aware of the threat at that point are not surprised, and those who are not aware, are surprised.
If the PC who goes first yells "look out", that doesn't mean that the other characters are going to instantly know exactly what threat to look out for, and since by ...
The module makes no mention of a Dexterity (Stealth) check, implying that finding the secret door could be enough to trigger the surprise sequence.
Given the low DC to spot the secret doors, the NPC's low passive perception, and that the NPC is occupied by another task, it's entirely possible that the players need not roll any dice, assuming they aren't ...
RAW? No, you cannot.
HellSaint's answer covers this well. If you declare a hostile action (like casting hold person), the game drops into rounds and turns:
Begin the next round.
What can the Alert character even do?
If the Alert character goes first...
...describe the environment as you ...
There is only one other non-homebrew item, but you are not likely to come across it; the Infiltrator’s Key.
While holding the key, you can use an action to cast one of the following spells from it: alter self, invisibility, knock, or pass without trace. Once a spell has been cast using the key, it can’t be used to cast that spell again until the next dawn....
Your third interpretation is the way surprise works by the rules.
The rules on surprise state (emphasis mine):
The DM determines who might be surprised. If neither side tries to be stealthy, they automatically notice each other. Otherwise, the DM compares the Dexterity (Stealth) checks of anyone hiding with the passive Wisdom (Perception) score of each ...
An encounter does not start, initiative is not rolled, and surprise is not determined, until some combatant has reason to attempt to initiate it.
For all combatants to be surprised, that must mean that no combatant has noticed a threat.
And if no combatant has noticed a threat, why is initiative being rolled?
There is an important distinction to be ...
Yes, the Player’s Handbook and Dungeon Master’s Guide give some some examples of threats
Page 182 of the PHB has a section on noticing threats. An example of a threat it gives is “ a stealthy creature following the group“. The creature is not posing an immediate danger but they could potentially ambush the party if they were not noticed. This tells us that a ...
No Surprise Here
jgn's answer covers the rules related to Dexterity (Stealth) and Perception (Wisdom), and does it well, so I won't rehash them. Instead, I present an alternative: surprise isn't really a factor in this situation.
Trying to be stealthy is all well and good, but we're talking about a forewarned enemy laying in wait for a forewarned party. You'...
What happens when the stealth check is failed
First lets look at the rules of surprise.
Surprise (Starter Set Rulebook 9)
The DM determines who might be surprised. If neither side tries to be
stealthy, they automatically notice each other. Otherwise, the DM
compares the Dexterity (Stealth) checks of anyone hiding with the
passive Wisdom (Perception) score ...
The question asks for the rules as written. So far as I'm aware, no text follows up on how to use properly the 1st-level Sor/Wiz spell nerveskitter [trans] (Spell Compendium 146–7), the 4th-level Sor/Wiz spell celerity [trans] (Player's Handbook II 105), or the 5th-level Sor/Wiz spell contingency [evoc] (Player's Handbook 213). The game just didn't go in ...
The supprise round would happen as normal with you being flat footed. Then you would roll initiative (at +5 from nerveskitter) but would act first in that round (due to Celerity) but would be dazed from that point. In following rounds you would act as per your rolled initiative.
The Nerveskitter description starts-
You cast this spell when you and your ...
The creature remains surprised.
There aren't any special rules on the Dread Ambusher or Assassinate features that would cause a creature to stop being surprised. Since there aren't any special rules, the creature remains surprised until the end of their turn as normal.
If you aren't sure what the surprised state is, please refer to this question:
At which ...
Adding on to MivaScott's answer, a Ring of Spell Storing might serve your purpose.
Ring of Spell Storing
This ring stores spells cast into it, holding them until the attuned wearer uses them. The ring can store up to 5 levels worth of spells at a time. When found, it contains 1d6 - 1 levels of stored spells chosen by the DM.
Any creature can cast a spell of ...
The question does correctly outline most of the direct benefits of an ambush. There are a couple of things to note, though.
Firstly, a number of Rogue abilities are keyed to working on flat-footed opponents. Consequently, Rogues are best able to take advantage of an ambush situation. In the same way, the 3rd level Rogue ability Deny Advantage will mean the ...
If everyone is surprised, no one is
It doesn't matter whether it's possible or not. There's probably enough room for a GM to decide that everyone in an encounter is surprised, but it's not an interesting situation. Let's look at what happens when the GM decides it happens:
Everyone rolls initiative
First player has a turn. They can't do anything but lose ...
By strict RAW of your question, situation 1 is correct. You have already rolled initiative, so you have to act in turn order. However, that isn't the only way your DM could have played this scenario.
Combat begins when the DM says so
There isn't any explicit guidance as to when combat encounters technically begin. We know what a combat encounter is, an ...
You are surprised if don't notice a threat
The rules for surprise state that you only have to notice a threat in order to not be surprised:
Any character or monster that doesn't notice a threat is surprised at the start of the encounter.
If you have successfully hidden then you are undetected
Even if a monster notices someone else in your party, if ...