How can I maintain control over a Dybbuk for over an hour?

Question

A Dybbuk has Magic Resistance (advantage on saving throws against spells and other magical effects), and can cast Dimension Door at will.

This means it can get out of a Magic Circle pretty quickly, it breaks the control from Summon Greater Demon quickly as well, or at least quick enough that you can't cast Planar Binding on it, even if you do know its true name.

Assuming it is summoned by Summon Greater Demon, how can its spellcasting be negated for long enough to bind it, or have a chance at binding it? Can it be summoned another way and controlled for over an hour? The lower the level of casters needed, and lower numbers of them, the better.

Answer 1

Just stop it from speaking

The best solution is a magic circle for the sake of planar binding (see this Q&A for solutions on casting planar binding on a creature conjured by a spell with a 1-hour duration if the Dybbuk was conjured by summon greater demon).

As you mention, dimension door is a problem since it can try to escape this way on a save (and can do so at-will):

If the creature tries to use teleportation or interplanar travel to [escape the cylinder], it must first succeed on a Charisma saving throw.

However, keep in mind that dimension door has a verbal component:

Most spells require the chanting of mystic words. The words themselves aren't the source of the spell's power; rather, the particular combination of sounds, with specific pitch and resonance, sets the threads of magic in motion.

...as such, any method you might use to restrict verbal components would prevent the fiend from escaping. The Basic Rules provide at least a few examples:

a character who is gagged or in an area of silence, such as one created by the silence spell, can't cast a spell with a verbal component.

If silence is your best solution (which it might be since a Dybbuk is an incorporeal spirit when not possessing a corpse), then you will need two+ level 3 spellcasters (bards, clerics, desert druids, and some warlocks are eligible) or a number of level 5 rangers to cast it six times.

For the spellcasters that have ritual casting, you only need two since they won't run out of spell slots (setting up a ritual takes 10 minutes, which is equivalent to the spell duration). For the others, you need enough spell slots.

For an option with a single caster, you might be able to have a druid, ranger, or trickery cleric use pass without trace in the vicinity of the Dybbuk:

A veil of shadows and silence radiates from you, masking you and your companions from detection. For the duration, each creature you choose within 30 feet of you [is affected by the spell effect]

It is not abundantly clear if the silence from this veil prevents verbal components in the same way as the silence spell but a generous DM might agree that it does. If so, you only need one casting since pass without trace lasts an hour.

Innate Spellcasting

Restricting verbal components works because the Dybbuk's stat block only removes the need for material components; Verbal components still must be used:

It can innately cast the following spells, requiring no material components

What if I don't have silence or enough spellcasters to cast it 6 times

In this case, PixelMaster, in the comments, offers some other solutions:

• a level 9 cleric or fiend warlock could cast hallow with Extradimensional Interference:

Affected creatures can't move or travel using teleportation or by extradimensional or interplanar means.

This option has the added benefit that you could cast the spell any time before summoning the Dybbuk since it lasts until dispelled (simply exclude fiends so that they can enter the area of effect). However, it does require material components costing 1,000 gp and a full day to cast.

• you could use dimensional shackles (a rare magic item):

In addition to serving as mundane manacles, the shackles prevent a creature bound by them from using any method of extradimensional movement, including teleportation or travel to a different plane of existence.

This may only work if the Dybbuk has possessed a corpse as it is otherwise incorporeal.

Answer 2

Polymorph it into a Knucklehead Trout (from IWD:RotF)

Disclaimer: First off this requires your GM to agree that Planar Binding can be cast on a Polymorphed Fiend, or a Fiend that is Polymorphed for some of the casting time.

The spell Polymorph specifies "a new form", and that "The target's game statistics, including mental ability scores, are replaced by the statistics of the chosen beast. It retains its alignment and personality", so it depends on whether your GM consider 'type' to be a Game Statistic - and if they do, whether your GM allows you to cast it on a Fiend that remains within range for the duration but spends some amount of time in the middle as a Beast, or even is still a Beast when the spell ends?

I'm going to be assuming that the Planar Binding spell works as long as the target is a Fiend at the start of casting OR that Polymorph doesn't remove the Fiend typing from the Dybbuk, with a note on what to do if it needs to be a Fiend at the end of casting, but this shouldn't be considered a flawless RAW solution.

Process

Before the main event, someone begins casting Magic Circle with a 4th slot, for 2 hour duration (1 Level 7+ caster), just in case.

Once Magic Circle is complete, 3 Casters all Ready spells, or ready the casting of spells for the one with a long cast time, with various triggers.

One (level 7+) will cast Summon Greater Demon as soon as the others are ready (this is basically just to ensure the turns happen in the right order).

One (level 9+) will start casting Planar Binding, with the trigger "When the Dybbuk appears".

One (level 7+) readies a Polymorph spell, with the trigger "When Planar Binding starts being Cast".

When one caster completes Summon Greater Demon they can issue it a verbal command that it must obey for at least one turn, as it makes the Charisma Saving Throw at the end of its turns. Therefore they can order it to accept the Polymorph spell readied, with the trigger "when the Planar Binding Spell starts to be cast".

The Dybbuk doesn't make the Saving Throw to resist Polymorph as it's a willing target, so the spell's caster polymorphs it into a Knucklehead Trout, which is the creature with the lowest Charisma Modifier I can find (-5), from Icewind Dale: Rime of the Frostmaiden, although the Quipper from the Monster Manual has a Charisma Modifier of -4 so is a close second.

While Polymorph is up, the Dybbuk can't cast spells, doesn't have it's Magic Resistance, and has such a significant modifier that a decently statted caster could maintain control over it (the control from Summon Greater Demon) in that form, as its max roll is (20-5), so 15. You should also just put it in a bucket of water, physically restraining it and keeping it from dying (which would revert it to it's normal form).

After 1 hour Planar Binding completes, and since Polymorph was cast after Planar Binding it can still be in effect.

If your GM considers the Polymorphed Fiend a valid target, it now makes a single Charisma Saving Throw, with the same -5 Modifier.

If they insist that the Dybbuk must be a non-polymorphed Fiend at the start and end of the spell, the caster of Polymorph can drop it just before the spell and it makes Planar Binding's save with a +2 modifier and advantage from Magical Resistance. If you've maintained control via Summon Greater Demon you could potentially try and order it to accept the Planar Binding and fail the Save but that's not a RAW option.

Answer 3

I'm surprised no one has mentioned this, but Private Sanctum blocks teleportation.

Also, glyphs of warding can be used to circumvent your concentration needs.