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The wizard in my campaign has the Eye of Vecna and is a yuan-ti, and this means he has suggestion and dominate monster at least once a day. Could he simply suggest to a demon that it reveal its true name? Or force it to?

If so, wouldn't that effectively give him permanent power over the demon, and make a spell like planar binding basically useless?

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@SevenSidedDie Thanks! – Nemenia Feb 12 at 19:26
up vote 8 down vote accepted

Yes, you can learn a demon's name this way, but it won't give you power over it.

Dominate monster could be used to compel a demon to reveal its true name, but the demon gets a saving throw (and most demons have Magic Resistance, so they will have advantage on the saving throw). Since revealing its true name is "obviously harmful" to the demon, suggestion would not be able to compel it in that way.

However, there is no rule that says that knowing a demon's true name gives the caster any special control over it. Planar binding only works on a creature that's visible within range. By knowing a particular demon's true name, a spellcaster can cast gate:

When you cast this spell, you can speak the name of a specific creature (a pseudonym, title, or nickname doesn’t work). If that creature is on a plane other than the one you are on, the portal opens in the named creature’s immediate vicinity and draws the creature through it to the nearest unoccupied space on your side of the portal. You gain no special power over the creature, and it is free to act as the DM deems appropriate. It might leave, attack you, or help you.

Note that knowing a creature's true name gives you no advantage on either dominate monster or planar binding.

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planar binding is meant to enslave a creature to your will. likewise, a demon's true name is able to control them. you can use gate on their "name" such as "Arcturious, dark general of gehenna" but not their TRUE name, which is a secret. I think you missed the point of the question – Nemenia Feb 12 at 19:42
    
hmm. so now im wondering, do you HAVE to find out a demon's true name to gate it, or can you use the demon's freely given name – Nemenia Feb 12 at 19:51
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No, I understood the question, and I understand the lore around true names of demons. Gate specifies that "pseudonym, title, or nickname"s don't work. Without that, gate just opens a portal to the Abyss for one minute; you may or may not get a demon that way. – Mark Cogan Feb 12 at 19:56
    
We're having this discussion in two places. I'm deleting my comments from down here to minimize duplication and I'll answer in my response above to you. – Lino Frank Ciaralli Feb 12 at 20:18
    
With the edits this has become more comprehensive than the original, so I'll accept this – Nemenia Feb 13 at 1:50

Yes, the rules for this are in the Monster Manual

Suggestion just requires a suggestion that sounds reasonable within a sentence or two. You could suggest the following as a very reasonable suggestion:

"I'll make you a deal demon. You tell me your true name so I can protect myself from you on the prime material plane, and I'll release you to wreak havoc on anything you please, excluding me."

In the MM pg. 53, there's an excerpt directly related to this, emphasis mine:

DEMON TRUE NAMES

Though demons all have common names, every demon lord and every demon of type 1 through 6 has a true name that it keeps secret. A demon can be forced to disclose its true name if charmed, and ancient scrolls and tomes are said to exist that list the true names of the most powerful demons. A mortal who learns a demon's true name can use powerful summoning magic to call the demon from the Abyss and exercise some measure of control over it. However, most demons brought to the Material Plane in this manner do everything in their power to wreak havoc or sow discord and strife.

However this also highlights that you would only have some measure of control, not permanent power. The follow up sentence in the description provided highlights that the demon will actively fight you on this, making Planar Binding a requirement to force something into servitude.

On a personal side note: This is why I use Devil's instead of Demons. Devil's don't like service, but they expect to be rewarded and will willingly follow commands when they are issued. Source is MM pg. 66, under the section Obedience and Ambition.

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well that sounds pretty solid. does it detail anything about what kind of power it gives or is it really that vague? – Nemenia Feb 12 at 19:45
    
@Nemenia I think it's referring to existing spells. – SevenSidedDie Feb 12 at 19:50
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Yes, the "power" is defined in the gate and planar binding spells. gate is "powerful summoning magic" and planar binding (or dominate monster) can "exercise some measure of control". – Mark Cogan Feb 12 at 19:54
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No. The true name lets you (via gate) get the demon close enough to cast planar binding. It doesn't help with actually performing the binding. – Mark Cogan Feb 12 at 19:58
    
so you still need to have very high level magic to get one for any length of time, even if you know the true name. alright, that seems fair. i think ill give it to this one – Nemenia Feb 12 at 19:59

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