On casting 'Imprisonment / Chaining', on changing &/or non-stable surfaces - where, where do the chains go?

Question

The 9th level spell Imprisonment can be cast a few different ways. One possible cast-version creates 'chains' that keep the target restrained:

You create a magical restraint to hold a creature that you can see within range. The target must succeed on a Wisdom saving throw or be bound by the spell; if it succeeds, it is immune to this spell if you cast it again. While affected by this spell, the creature doesn't need to breathe, eat, or drink, and it doesn't age. Divination spells can't locate or perceive the target.

Chaining. Heavy chains, firmly rooted in the ground, hold the target in place. The target is restrained until the spell ends, and it can't move or be moved by any means until then. The special component for this version of the spell is a fine chain of precious metal.

Note that these special Imprisonment-Chains are firmly rooted in the ground (R.A.W. / above). Here are my variants on one concern, below:

• what if the target is 60' to 120' up... on a wooden floor? Do the chains reach all the way to ground level as the spell takes effect?

• what if the imprisoned is standing on any firm yet non-stable surface when this spell is cast? Examples abound: on a ship, in the air due to a kite or hot air balloon. Or even on an otherwise quasi-stable surface such as sand or ice or swamp or lava or glass - the chains would have to take root somewhere 'in the ground'.

I admit that this question isn't deeply serious. Still, I am darn curious about where these crazy chains would end up going.

Answer 1

The chains need to hit the ground

As a powerful and thematic 9th level spell, imprisonment should probably be thought of more as a plot device than as a normal spell. Still, we can make some inferences about how the spell works in absence of easily accessible ground.

First, we know that all parts of the spell are game rules. Not only are the chains literal, they are to be rooted in something that common English would refer to as "ground." This probably rules out ships, hot air balloon chassis, and other things that one would more properly call "floor."

The ground is not the spell's target, but must be accessible

So, how do we know that the ground is close enough? Most spells (such as earthquake) that affect the ground specify that the point in question must be within range, because they are indicating a target. One exception is destructive wave, which requires that the caster "strike the ground." The range of this spell is self, which the Player's Handbook tells us indicates that the target is the caster.

From this, we can infer that spells can require the presence of ground within a certain range, without that area of ground being considered the spell's target. In the above case, the caster must clearly be able to strike the ground, which should require that the ground be close enough to strike.

The spell is still limited by its range

That brings us to imprisonment. As with destructive wave, this spell requires that we have access to "ground." Unlike destructive wave, it does not imply a distance in the text alone. We know that a spell's effects can extend beyond its range in some cases. Does that apply here? Going back to the PHB, we see that there are two reasons a spell can effect things past its range: either it affects an area that partially extends beyond the range, or the effects take place after the spell has been cast. Neither of these reasons is applicable here, since imprisonment does not affect an area, and we are concerned with the moment of casting.

Look back at destructive wave. The spell's range is self, so by default it only affects you. However, since specific overrides general, the text of the spell allows it to affect others by specifying a range within which that is possible. Thus, the spell's effects extend past its range.

Imprisonment has no such exception, thus we return to the general rule: a spell's effects upon casting are limited to its specified range.

The chains probably need to hit the ground within 30 feet of the caster

It's worth noting that this is not a good way to think about imprisonment. When this spell is cast, it is ending a story, or beginning one. While I believe this is likely the Rules as Written answer, examining spells and game effects like this one with so mechanical a lens is dangerous. Use this way of looking at the narrative at your own risk.

Answer 2

The spell does what it says, and it doesn't mention the chains having a maximum range-to-ground, so it seems to me that the chains would reach all the way to the actual ground, whether that's within the spell's range or not, and it's up to the DM to decide exactly how that manifests if you're using it in a strange situation.

Given this is a 9th level spell, which often have intentionally bizarre effects that put enormous stress on the game's rules in order to do something really cool, my instinct is to ignore the strict rules and go as cinematic as I can. I'd describe the chains wrapping around the target and dragging them down to the ground regardless of what's in between, so you get to have the villain crashing through multiple floors of a building on their way, or dragged under an icy lake. (They'll be fine, they don't need to breathe while imprisoned.)

Answer 3

Rule 0 Applies here I'd imagine.

Maybe they go infinitely far until they find the closed anchor point but the caster can only target things in the spell range for reasons.

Perhaps there is enough chain that it can form a solid surface on unstable ground, ie sand?

Or quite literally It's magic, I ain't gotta explain **** (TVTropes Warning).