How does Rune Knight's Fire Rune interact with a Succubus' Etherealness ability?

Question

Had a run in with this one in tonight's game. I as the Rune Knight fighter used my fire rune on the succubus that kept running away, thinking I would continue to restrain her. The text reads:

When you hit a creature with an attack using a weapon, you can invoke the rune to summon fiery shackles: the target takes an extra 2d6 fire damage, and it must succeed on a Strength saving throw or be restrained for 1 minute. While restrained by the shackles, the target takes 2d6 fire damage at the start of each of its turns. The target can repeat the saving throw at the end of each of its turns, banishing the shackles on a success. Once you invoke this rune, you can't do so again until you finish a short or long rest.

Strength save or be restrained, got it. However, the succubus has the ability to go to the Ethereal plane:

Etherealness. The fiend magically enters the Ethereal Plane from the Material Plane, or vice versa.

Does the fiery shackles follow the succubus to the Ethereal plane, or do they remain in the material?

I've found arguments for both. They could follow to the ethereal plane because any teleportation with a range of self also brings any items worn or carried. If the shackles are still on her, she would still be restrained and still be dealt fire damage.

However, there are multiple spells that specify that once in the ethereal plane, no effects that originate from the material plane affect them. For example, blink specifies:

While on the Ethereal Plane, you can see and hear the plane you originated from, which is cast in shades of gray, and you can't see anything there more than 60 feet away. You can only affect and be affected by other creatures on the Ethereal Plane.

And the spell Etherealness, which shares the name as the succubus effect, states:

You ignore all objects and effects that aren't on the Ethereal Plane, allowing you to move through objects you perceive on the plane you originated from.

But both of these have the issue of where does the effect from the fire rune originate? If from the shackles, and the shackles are teleported with you, then it would continue. However if the player is the source, then it wouldn't.

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In the moment, the DM stated that the shackles dropped and the creature could move freely. I was kinda distraught since I thought that it would stop her, but I'm curious how others would rule it.

Answer 1

The "fiery shackles" are not an object

The "fiery shackles" are not a tangible item that exists on the target of the Fire Rune; it is the flavor text that explains the effect. The effect specifies the condition for escaping it: making a Strength saving throw.

If teleporting or leaving the plane of existence were enough to end the effect, it would say so. By a strict reading of the rules, the Succubus would still be under the effects of the shackles even after using its Etherealness trait.

Moving to the Ethereal Plane is not the same as casting the etherealness spell

If the effect functioned as the etherealness spell or the blink spell, it would specify so. Instead, it has its own effect which describes exactly what it does. Therefore, none of the principles of those spells apply to this effect.