I'm about to set a very important precedent tonight - Dragon Heist Spoilers

[Bjarkmundur]

Tonight my group is knocking on the doors of the Cassalanters. There's a good chance Victoro will cast a 8th level Dominate Person spell on one of the characters. My player characters are not looking for a fight, they don't even know how the Cassalanters are involved. They are just going to go and say hi.

This sets an important precedent regarding spellcasting in a social situation. The caster obviously provides a a verbal and somatic component, and probably tries to blending the components into whatever story he's telling. For concealing the components, I'm guessing it's just a Charisma (Deception) vs. Wisdom (Insight), or is it an some other check to recognize the phrase? If he manages to make the gesture and magical phrase without raising suspicion, are there any other clues that a spell has just been cast?

Do I roll the wisdom save for the player I Victoro manages to pull of a 'subtle' spell, or does the player roll it?

Also, what are some interesting punishments I can use, once the dominated target gets the group arrested?

[firelistener]

Normally, I'd go with an Arcana check for recognizing a spell, with advantage to someone that shares a spell list with the caster. To notice the casting at all if it's subtle, I'd use passive perception. If you use passive insight, that'll be good too, I just only bother with perception in my games.

So it's DM rolls the passive check to notice a spell is being cast, player rolls subsequent check to tell what spell it is.

[Hail Tempus]

Spellcasting is apparent to anyone within sight of the spellcaster. The only exception is if the caster is a sorcerer with subtle spell.

Take a look at the description of somatic, verbal and material components in the PHB. From all indications, there's no way to cover up that spellcasting is occurrin.

[Spiritchaser]

Caveat: I am not familiar with this adventure or circumstances surrounding this encounter

That said

I would be exceedingly reluctant to allow a character to do this unless the party was very heavily distracted and there were loud background noises that could reasonably sound like chanting a verbal component. Edit: perhaps a place where other spells were being cast? Even then there’s the somatic component to hide... it really doesn’t work.

If you really want this character to be capable of casting subtle spells, give them subtle spell meta magic and a small handful of sp.

[SanguisAevum]

Tonight my group is knocking on the doors of the Cassalanters. There's a good chance Victoro will cast a 8th level Dominate Person spell on one of the characters. My player characters are not looking for a fight, they don't even know how the Cassalanters are involved. They are just going to go and say hi.

This sets an important precedent regarding spellcasting in a social situation. The caster obviously provides a a verbal and somatic component, and probably tries to blending the components into whatever story he's telling. For concealing the components, I'm guessing it's just a Charisma (Deception) vs. Wisdom (Insight), or is it an some other check to recognize the phrase? If he manages to make the gesture and magical phrase without raising suspicion, are there any other clues that a spell has just been cast?

Do I roll the wisdom save for the player I Victoro manages to pull of a 'subtle' spell, or does the player roll it?

Also, what are some interesting punishments I can use, once the dominated target gets the group arrested?

According to the RAW, The casting of spells is inherently obvious to anyone who can see or hear the caster.
The only way around this is with a specific sorcerer meta-magic feat designed to circumvent this rule (Subtle Spell)
if you allow other spellcasters to do this, you are essentially invalidating that specific class ability.

Short answer... they will know he is casting a spell, but they might not know what... (there are optional rules for recognising spells as they are cast in xanthars)

[sithlordnergal]

So, I would go for the Deception vs. Insight check, with advantage given to casters. Personally, I always ignore the RAW thing that says spells can't sneakily cast without Subtle Spell. If a player or NPC wants to sneakily cast a spell, they are certainly allowed to try. It makes things more interesting for players and allows for things like Dominate Person to throw the players off a bit.

[Spiritchaser]

So, I would go for the Deception vs. Insight check, with advantage given to casters. Personally, I always ignore the RAW thing that says spells can't sneakily cast without Subtle Spell. If a player or NPC wants to sneakily cast a spell, they are certainly allowed to try. It makes things more interesting for players and allows for things like Dominate Person to throw the players off a bit.

I would argue that if you were going to do this, you’d really want to clarify it in session zero.

This change goes to the core balance of spell casters vs. martials, and strongly favours classes which, in general, to not need any help. This has the potential to come up lots, and even be critically defining in social campaigns and stealthy campaigns. That doesn’t even touch on potential secondary fallout you may indirectly create from this precedent.

If it’s out there in the open at session zero, well, honestly fair enough, but changing a fundamental about how magic works part way through Shouldn’t be done without player buy in, and that would kind of give the game away in this case.

[GreyBlack]

Does the spellcaster in question have subtle spell? If no, you're right. This is an important precedent that your players can mimic a class feature using skills.

[sithlordnergal]

I would argue that if you were going to do this, you’d really want to clarify it in session zero.

This change goes to the core balance of spell casters vs. martials, and strongly favours classes which, in general, to not need any help. This has the potential to come up lots, and even be critically defining in social campaigns and stealthy campaigns.

If it’s out there in the open at session zero, well, honestly fair enough, but changing a fundamental about how magic works part way through Shouldn’t be done without player buy in, and that would kind of give the game away in this case.

Oh I always let my players know of my changes and house rules at session 0. And I haven't really found that it unbalances things too much. At worst it gets players to invest in Deception and Insight, and players tend to get paranoid whenever I start rolling stuff. I find what I gain from being able to quietly charm a party member without the rest of the party knowing to be well worth any potential balance issues.

EDIT: Given I tend to run things over Roll20, I can very easily whisper to a player to keep the rest of the table from finding out. I've done it with Dominate Person before to great effect. They were in an area with an Elder Oblex, I called for initiative, the party couldn't find the hidden Oblex, and it Dominated the Paladin. So it took over said Paladin and had them Smite the Wizard with everything they had. It made a dangerous encounter even more interesting because they had to deal with both the Paladin, and a hidden Oblex that was controlling their ally.

[firelistener]

I really disagree with the idea that allowing sneaky casting attempts is the same as Subtle Spell. Subtle Spell lets you cast without V/S components so you can still do things while bound and gagged or cast a spell without anyone hearing/seeing you cast at all.

[Spiritchaser]

Oh I always let my players know of my changes and house rules at session 0. And I haven't really found that it unbalances things too much. At worst it gets players to invest in Deception and Insight, and players tend to get paranoid whenever I start rolling stuff. I find what I gain from being able to quietly charm a party member without the rest of the party knowing to be well worth any potential balance issues.

While I don’t personally like that change, I certainly think you’ve gone about it in the best way. Myself, if I want a foe to be able to have that ability, I make them sorcerers with subtle spell.

Regardless: In the case presented in the OP, the situation is rather different

The players have not had this explained in session 0

Not only are there the balance issues, there’s also a blind running change to the rules

[Keravath]

Tonight my group is knocking on the doors of the Cassalanters. There's a good chance Victoro will cast a 8th level Dominate Person spell on one of the characters. My player characters are not looking for a fight, they don't even know how the Cassalanters are involved. They are just going to go and say hi.

This sets an important precedent regarding spellcasting in a social situation. The caster obviously provides a a verbal and somatic component, and probably tries to blending the components into whatever story he's telling. For concealing the components, I'm guessing it's just a Charisma (Deception) vs. Wisdom (Insight), or is it an some other check to recognize the phrase? If he manages to make the gesture and magical phrase without raising suspicion, are there any other clues that a spell has just been cast?

Do I roll the wisdom save for the player I Victoro manages to pull of a 'subtle' spell, or does the player roll it?

Also, what are some interesting punishments I can use, once the dominated target gets the group arrested?

This kind of depends on how casting magical spells works in your world. RAW, I think everyone knows a spell is being cast, the verbal and somatic components are distinctive enough that most folks know a spell is being performed. They won't know what spell it is though. The sorcerer subtle spell meta magic is so useful because it removes the need for verbal and somatic components so others can't interrupt your spell casting (e.g. counterspell).

Personally, I wouldn't set a precedent of being able to cast a spell without it being noticed.

However, presumably an NPC with an 8th level spell slot is reasonably bright. They are meeting the PCs in their home or place of business, the PCs are investigating something, he sounds like an important individual - he would likely have a parlour or waiting room where the PCs would wait until he is available to meet with them. If the NPC already knows he wants to dominate one of the PCs he would have them wait in the parlour, observe them through a spy hole, cast the spell where they can't see or hear it being cast and THEN meet with them allowing his dominated puppet to respond naturally not alerting the rest to his status. On the other hand, keep in mind that the PC may MAKE his saving throw and not be dominated. Your NPC should have a plan B since it would be unfair to force your players down a railroaded path if they make their save.

Also, the spell doesn't indicate either enmity to the caster or other side effects. If they make their save they are likely completely unaware something tried to dominate them. If they fail, they hear a voice they must obey. When it ends, all they know is that they started hearing voices. The caster could possibly even wait for some time until long AFTER the character has left before exerting control so that it may not even realize which encounter resulted in the domination. However, any damage received allows for a new saving throw so it would be best if the character caused problems that won't result in damage.

[Contrast]

Probably the most elegant solution would be to see if one of the players separates themselves from the others (perhaps they are invited for a private audience?). They can then be swooped upon without worrying about hiding spell components.

[ProsecutorGodot]

Victoro doesn't risk having their would be pawn's allies immediately become aware of the domination. If the situation doesn't come up where he couldn't safely cast the spell without a witness he probably wouldn't do it to begin with.

It says as much in his description:

Victoro Cassalanter reacts with fiery rage to the sight of uninvited guests. He demands to know their names and calls for the butler, Willifort Crowelle, to show them out. If he has the chance, he subtly casts dominate person on one of the characters using an 8th-level spell slot (extending its duration to 8 hours) to plant a spy among their ranks.

If no chance to do it subtly shows up, he doesn't do it. He likely wouldn't suffer any consequences doing it in the open but the spy would be entirely ineffective in reporting the parties crime if the remaining party members can attest truthfully to the reporter being charmed to do so. If Magistrates are made aware that the reporting member was charmed they probably won't believe the party actually committed the offense. At worst they'd be detained and questioned for a bit but not directly imprisoned as their rights have been infringed upon (casting spells against someone is a serious offense).

Ammalia on the other hand has no issues casting Charm Person on up to 5 creatures in clear view, the module makes clear that the characters would know this is illegal but nothing will come of it because of their families influence.

[Damon_Tor]

1. PCs arrive at the NPC's home, it takes a while for them to see the NPC in question (just a long time waiting at the door, maybe interference run by butler or another household member, etc).
2. Made aware of a potential threat, the NPC casts "Silence" on the part of the room where the NPCs are to meet him.
3. PCs are led into the area of Silence, where they see an illusion of the NPC sitting behind a desk. (a chance the PCs can spot the fake with an Int save, as is typical for illusions). They will surely notice the uncanny quiet in the room.
4. The NPC speaks with them telepathically (via spell or another item) excusing the use of a silence spell as a necessary precaution against unknown spellcasters. If the PCs notice the illusion and mention it he makes a similar excuse.
5. The illusion of the NPC changes to something very interesting to the PCs: perhaps a clue vital to their quest, a map of waterdeep with the possible locations of something important marked in red.
6. Engage the players by asking for skill checks related to deciphering the information being fed to them by the clue. This is really an opportunity for them to do something else, to look around the room instead. Give the players one last chance to avoid what's coming.
7. While they are distracted by this information, the real NPC appears behind them and casts Dominate Person on the PC he thinks will be most susceptible. If one of the PCs opts to spot-check the room instead of looking at the map they can possibly beat the NPC's stealth check and see him, though it's worth noting that they will be disadvantaged while doing so due to the silence spell.
8. This ends his concentration on the Silence spell, and the PCs will likely notice a difference in the sounds, as the uncanny silence gives way to ambient sounds.
9. He greets them in person, vocally this time. He'll tell them he's dropping the illusion and silence spells as a show of good faith. He'll ask a favor of them at this point, maybe just for them to keep him in the loop on their progress. Mostly the function of asking the favor is to help avoid suspicion: someone who asks nothing of the PCs is always hiding something, after all.

Because they are under the effect of Silence, they won't be able to hear the verbal components of the Dominate spell, and as long as all of them remain focused on the clue being shown to them as a distraction (or fail to beat his stealth check) they shouldn't see the gestures either. Ultimately he'll come off as an overcautious, perhaps paranoid, spellcaster, but as long as the intel he feeds them is good they should feel like he's reasonably trustworthy.

[J-H]

He's a high level NPC casting a spell in a social situation. Just give him an item that lets him use Subtle Spell at a cost of 1hp damage (non-mitigatable) per level of the spell cast. Then the party gets passive perception vs. his Stealth check or Deception check, and maybe they notice that this rich wealthy guy is randomly bleeding or something.

[MikeRoxTheBoat]

Just give him a custom magic item? "Requires Attuenement by a spellcaster. While holding this item you can cast a spell without verbal or somatic components. You can do this once per long rest."

Just be prepared to have a player get that item if they kill the guy.

[Wizard_Lizard]

for the somaic component, probably sleight of hand vs perception.

[Doug Lampert]

Spellcasting is apparent to anyone within sight of the spellcaster. The only exception is if the caster is a sorcerer with subtle spell.

Take a look at the description of somatic, verbal and material components in the PHB. From all indications, there's no way to cover up that spellcasting is occurrin.

It's his house, he's a noble, it's a party, he has his staff on hand. He waits for one of them to go to the urinal and hits him there.

It's non-trivial to come up with a better situation than this for him to be able to separate them and cast on one in isolation.

You do not need to introduce a rule for hidden spellcasting when the gestures are supposed to be intricate and clear and the voice loud and clear.

[ProsecutorGodot]

I'd avoid creating any custom magic items, it's only going to have your players wanting it for themselves down the line.

Easiest solution: Willifort informs the party that Victoro will see one of them personally in his office, the rest are either escorted to the Smoking Room or asked to remain outside the manor, their choice. Normally he would see them all in the Smoking Room but he's rather busy today.

This accomplishes several things:
-It should be a big red flag if the party was at all suspicious of them before, gives them time to reconsider this visit
-It gives the one party member some time to observe for any potential details in Victoro's office
-The remaining party members can attempt to find clues in the Smoking Room or explore elsewhere if they have the means
-If you're able, the party member being chosen can actually be pulled aside from the rest of the group for this chat to better facilitate them not knowing that they were dominated

I just don't see any need to create a situation where he has to do this in front of the entire party. They're only entering the manor with his permission to begin with assuming you're going to let them in at all, meeting with Noble's isn't generally your choice. He's got plenty of time to create a situation where he doesn't have to risk exposing himself as a shady character.

[Laserlight]

If he doesn't have Subtle Spell, he has Obvious Spell. In that case, if he wants to Dominate someone unbeknownst to the rest, he needs to get the target away from the rest, or announce "I and my spellcasting servants are casting Prestidigitation and Minor Illusion for the special effects!" and them hope nobody with Arcana skill is paying attention, or something of that sort.

If you let a villain do it (either by himself, or with a cheesy item), then the PCs will be doing it too; and a PC with Subtle social spells and CHA skills can run roughshod over your plans.