Preventing a Monster from using Etherealness

Question

So lets say you get the jump on a monster/spellcaster and are able to bind their hands and gag them before they have a chance to react. My question is if that would also prevent them from using the "Etherealness" action listed in their monster stat block. Here it is:

Etherealness: The hag magically enters the Ethereal Plane from the Material Plane, or vice versa. To do so, the hag must have a heartstone in her possession.

I assume that the partial interpretation is that the Etherealness monster action behaves the exact same way as the 7th level spell of the same name. However, does the monster action have any sort of Verbal, Somatic or material component that would be rendered impossible if they are bound/gagged?

If it was a hag in this case, would they have to be holding the Heartstone, or just have it on their person? Is it up to DM discretion?

Answer 1

You must prevent a Hag from taking an action, or remove the Heartstone from their possession, in order to stop them from becoming Ethereal.

Spellcasting vs Actions

The Hag has an ability to take the action "Etherealness" as quoted in your question. The description does not state that this is spellcasting, but only requires their ability to take an action and to be in possession of the heartstone.

Had this been spellcasting, it would have been listed under at-will or innate spellcasting within their stat block (either the Hag's or if using the Variant rule for Hag Covens) and would require somatic and verbal components (see below.)

Holding vs Having

As long as the Heartstone is possessed (not held, just has) and the Hag can still take an action (doesn't have the condition Incapacitated), they can become Ethereal.

This is similar to the idea that you can have a shield in your possession, but that doesn't give you the benefits of the shield. In this case, possession is all that's necessary. Had the Monster Manual said "hold" or "wield", it would have required that.

Innate spellcasting and components

Whether or not components are required for a creature's innate spellcasting will be determined by their stat block. A Hag does not require material components to cast their spells, but do require somatic and verbal. A bound and gagged Hag would be unable to cast their spells that require those components.

If you are thinking about whether or not a bound/gagged creature has been Incapacited, consider that conditions like Grappled and Restrained do NOT remove the ability for a creature to take an Action. Being Stunned, Paralyzed, or Unconscious are the types of conditions that prevent Actions.

Preventing Etherealness as an Action

The best way to do this in the case of the hag is to remove that Heartstone once the creature has been bound/gagged (or better...before!) Otherwise, not only do you need to restrain and gag them, but you will also need to make sure they are either Stunned, Paralyzed, or Unconscious by either magic or reducing their HP to 0.

Answer 2

Bound and Gagged Would not Work

As was said in NautArch's answer, the action is not the same as the PHB spell, it is an innate spell. It doesn't require spell components. It only requires the hag to have their hearthstone in her possession. The takeaway is that bounding and gagging wouldn't work. You'd have to take the hearthstone away from her; which would be the "simplest" and least resource intensive way to prevent her from using Etherealness. Simplest doesn't mean easiest.

Spells

Easy Spells

Hold Monster would also work. However, Suggestion and Dominate Monster wouldn't work, as the Hag is immune to charm effects.

Hold Monster

Choose a creature that you can see within range. The target must succeed on a Wisdom saving throw or be paralyzed for the duration.

Spells that prevent the creature actions would also work. A number of conditions that prevent actions like things that cause the Hag to become unconscious, incapacitated, Paralyzed or petrified. Sleep comes to mind.

Sleep

Starting with the creature that has the lowest current hit points, each creature affected by this spell falls unconscious until the spell ends

Big Long Spells

Magic Circle can be cast to prevent fiends and fey from leaving the circle. The casting time of a minute makes this not a fast thing you can just do in the heat of battle, but you could set a trap pairing the circle with Glyph of warding or the like.

When you cast this spell, you can elect to cause its magic to operate in the reverse direction, preventing a creature of the specified type from leaving the cylinder and protecting targets outside it.

Planar Binding would allow you to take control of the Hag, as Hags are Fey, except the Night Hag who is a Fiend, but all qualify for the spell. Then, once under the spell, you can command them not to leave. The problem is the long casting time and high level, and again, it would have to be planned out ahead of time.

A bound creature must follow your instructions to the best of its ability. You might command the creature to accompany you on an adventure, to guard a location, or to deliver a message. The creature obeys the letter of your instructions...

Answer 3

By RAW, you wouldn't be able to prevent them from using Etherealness.

It's just a thing that the hag can do, as long as they have the heartstone on their person. And even then, one might argue that "in their possession" could just be as simple as being nearby in a location the hag controls.

Remember that monsters' abilities and PCs attacks/spells will sometimes follow different rules.

Those stat block abilities don't necessarily require components in the same way a PC's spells do.