Prevent a dragon from reverting shape

[EggKookoo]

So let's say I (as the DM) want to create a one-use magic item that can be used to prevent a metallic dragon in human form from reverting to his dragon form. The dragon's Change Shape feature is magical, but not a spell per se. So counterspell wouldn't stop it in action. It seems like antimagic field would have the opposite effect, since the dragon's human shape is magically-generated.

I can just handwave a device that does exactly what I need it to, and I'm fine with that if that's the only way to really pull it off. But is there a near-RAW way to do this otherwise?

[Unoriginal]

So let's say I (as the DM) want to create a one-use magic item that can be used to prevent a metallic dragon in human form from reverting to his dragon form. The dragon's Change Shape feature is magical, but not a spell per se. So counterspell wouldn't stop it in action. It seems like antimagic field would have the opposite effect, since the dragon's human shape is magically-generated.

I can just handwave a device that does exactly what I need it to, and I'm fine with that if that's the only way to really pull it off. But is there a near-RAW way to do this otherwise?

1. Creating magic items with effects that don't follow spells isn't handwaving, it's the rules.

2.Could just have a normal collar, except it has the Indestructible minor magic item properly (making it a Common magic item, per the rules). Could also add an effect making so the collar can't be affected by transformation effects.

That way the dragon *could* shape change... If they don't mind being decapitated.

[EggKookoo]

1. Creating magic items with effects that don't follow spells isn't handwaving, it's the rules.

2.Could just have a normal collar, except it has the Indestructible minor magic item properly (making it a Common magic item, per the rules). Could also add an effect making so the collar can't be affected by transformation effects.

That way the dragon *could* shape change... If they don't mind being decapitated.

#1: Yeah, I know it's not really a problem. I like to use existing mechanics/features if possible.

But I like #2. I've been trying to visualize it as a restraining device, so that would fit.

[Corran]

A generous interpretation of Nystrul's magic aura, which you'll combine with something else (eg turn undead/unholy/faithless) might work. Though I would be hesitant to call this RAW. Any spell/feature that denies/restricts actions technically works (eg fear, etc), but what I can think of targets saves (legendary resistance) or does not let the dragon stick around.

I'd start looking at Eberron stuff. Not only 5e stuff specifically, but also lore from previous editions. Shapehanging dragons is a little more common as a thing in Eberron I think, so you might find a good idea for a gismo/spell/feature in Eberron stories. Not likely you'll find anything like that for 5e specifically, but you might find an idea that you would like enough to consider if you end up homebrewing something.

[Bohandas]

The closest thing I can think of offhand is the Cage of Kind spell from Call of Cthulhu D20, which does what you want but for a different kind of creature (it targets lycanthropes rather than dragons). It does however let us know what level a spell that does this should be, specifically it should be level 4.

[Spiritchaser]

So let's say I (as the DM) want to create a one-use magic item that can be used to prevent a metallic dragon in human form from reverting to his dragon form. The dragon's Change Shape feature is magical, but not a spell per se. So counterspell wouldn't stop it in action. It seems like antimagic field would have the opposite effect, since the dragon's human shape is magically-generated.

I can just handwave a device that does exactly what I need it to, and I'm fine with that if that's the only way to really pull it off. But is there a near-RAW way to do this otherwise?

I like the idea of a device that locks a creature into a form, and it could have some interesting history in your campaign, plus some cool backstory all its own... perhaps it was originally created to protect its wearer (or their companions) from undesirable shape shifting? Maybe someone would recognize it based on stories of such a thing...

Could the device be used on another creature or is it somehow permanently attuned to the dragon?

If it is transferrable, what would this do if you put it on someone temporarily polymorphed?

Interesting...

[EggKookoo]

Could the device be used on another creature or is it somehow permanently attuned to the dragon?

If it is transferrable, what would this do if you put it on someone temporarily polymorphed?

My initial intent was that it's meant for one use against this specific dragon (in universe). I know my players, though, and if the dragon busts out of the device and the PCs somehow survive all this, someone's going to pick up the pieces and try to see if they can derive some general use out of it. Which is partly why I'm looking to existing spells or mechanics for the effect. At the same time, if the dragon breaks free and escapes, and the PCs get ahold of the device, they would be happy with having something they could use only on him if they ever encounter him again.

To the second question, it depends. Does the device prevent shapechanging? Or as with Unoriginal's suggestion, does it merely not accommodate it, with potentially lethal results? (I lean toward the latter.)

[Anymage]

Despite having the Change Shape power, dragons are not shapechangers. You could polymorph one into a dog and leave it that way, no problem. It won't have the ability to shift back to dragon form. (It could very easily have some voice in the back of its head compel it into dangerous situations, to take advantage of the spell ending if it hits 0 HP. A collar of polymorph that causes you to die if your new form's HP hit 0 would be less of a stretch from established rules, and the fact that it's an item that's hard to slap on someone in the middle of a fight makes it less of a risk.)

There's also the part where RAW Polymorph only turns a subject into a beast, but that again is something an enterprising mage could very well create an alternative to in universe. And if the dragon breaks the collar and the PCs try to get ideas from it, being able to stick a defeated enemy in a lesser form is both the sort of humiliating thing that some archmages might love to do, and something that you only apply after you beat them so it's less likely to affect combat balance.