Amazing how many massively powerful creatures still don't have a high enough Athletics mod to escape these. 

Time to handcuff Orcus.

It says the shackles prevents a bound creature from using any method of extradimensional movement, but does that mean that the bound creature can't be the target of a interdimensional spell such as banish?  Do these work like a dimensional anchor that doesn't allow ANY interdimensional movement or does it simply disable the bound creature from activating movement/transportation/teleportation spells?

Specifically for my campaign: Would this prevent someone who is shackled from being thrust out of a pocket plane - say the interior paradise of a Rod of Security?

"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. They don't prevent the creature from passing through an interdimensional portal."

So you can't *move* extradimensionally, but normal movement can still get you through dimensions. If there is a gaping hole between different dimensions and a peaseant with only peasant allies and a single pitchfork could step through and land in a different dimension, the manacles won't stop it. If some teleporting effect is cast by or onto you, the shackles block it. The rod teleports you to a safe space, so it'll get blocked.

A little late but I would say yes on the banishment part. I was thinking the same thing when I saw this

My "Urban Bounty Hunter" character will need to grab these at some point...

Have to keep those pesky spellcasters from "bamfing" away.

But what action is being used to lock them?  A magical incantation?  A physical key?  A prayer?  Basically... how does one free a creature from the shackles?  

Either as the person who placed the shackles on the target, or as an adventurer looking to set free a Demon bound by them?

Disintegrate?  DC30 lockpick?  Dispel magic (what level?)?

What if you put them on a shapeshifter? Say my party puts a pair of magic handcuffs on Strahd and he turns into a cloud of mist?

Well, the shackles say that they 'prevent a creature bound by them from using any method of extradimensional movement, including teleportation or travel to a different plane of existence', not they 'prevent a creature from using magic to move'. So no, they would not prevent Strahd from becoming a mist cloud

Hmm. They would potentially prevent you from being snatched away by the mists though. Unless the mists count as a portal [shrug]. I could see a hyper paranoid Strahd survivor shackling themselves if they had an inkling that he'd return and try to snatch them up again for revenge

I would rule that the shackles (as a magical item) will reshape itself to fit whatever form the creature takes. So if they're a shifter, they'll still be shackled. If Stahd turns into mist, the shackles would go with him since he's effectively wearing them.

Good luck incapacitating Orcus!! :)

Could these be used to avoid the affects of the Donjon card in the Deck of Many Things since it prevents all dimensional travel?

bool

What if you polymorphed a creature that was wearing these. Would they basically become free as the gear is "melted into them" but when the polymorph is over they become trapped again.

Quote from Anoel >>

Amazing how many massively powerful creatures still don't have a high enough Athletics mod to escape these. 

Time to handcuff Orcus.

 Late reply, but never too late :P

As a DM, I give ad hoc Athletics proficiency or expertise to many creatures for which it would make sense.

Must say, this item is a good companion to my ioun stone of vocal disruption.

Quote from smitchen >>

What if you polymorphed a creature that was wearing these. Would they basically become free as the gear is "melted into them" but when the polymorph is over they become trapped again.

That'd be up to the DM. Some DMs would likely consider the shackles outside the boundaries of "worn or carried items", as it isn't something that isn't worn or carried per se but rather something extraneous to the creature (imposed from the outside). The shackles' magic might take into account the possibility of shapechange and polymorphing in a universe where such magic exists. But if the creature polymorphs into a creature without arms or legs, or a creature of Tiny size, then the shackles become of little use...

Solution (if need be): anti-transmutation dimensional shackles, wondrous item, very rare.

Technically yes, however your DM may choose to say no as in many worlds the cards were made by gods and supersede mortal magic items. 

This and momentary stasis from chronurgy wizards and you just become the magical time police

"The shackles adjust to fit a creature of Small to Large size"
Orcus is huge, so that doesn't work.

Quote from The_AZN >>

It says the shackles prevents a bound creature from using any method of extradimensional movement, but does that mean that the bound creature can't be the target of a interdimensional spell such as banish?  Do these work like a dimensional anchor that doesn't allow ANY interdimensional movement or does it simply disable the bound creature from activating movement/transportation/teleportation spells?

Specifically for my campaign: Would this prevent someone who is shackled from being thrust out of a pocket plane - say the interior paradise of a Rod of Security?

 "prevent a creature bound by them from using any method of extradimensional movement"

I think the key word here is "using". Implying that a creature that has the ability to extra dimensionally move, can't do so one their own accord. It does not say it can't be moved by someone or something else.