Spellcasting in reverse gravity - Concentration check

[Just a Guess]

I am preparing an encounter with an opponent who can cast reverse gravity and I am wondering what kind of concentration check it is to cast while affected by it.
Looking at the concentration check rules I found vigorous motion and violent motion. Which of the two would you say is better suited for the task at hand?

The motion created by reverse gravity may not be very violent but you are unable to control your movement and facing by "oscillating slightly" in the free air. (Assuming a ceiling higher than 10ft)

Sins of the saviours spoiler:

The players will be in a big room in a dungeon protected by a glabrezu demon. It can cast reverse gravity at will and has a space of 15ft (height of 18ft), so while no ceiling height is given I'd assume at least 20ft for the glabrezu to fit in. So if the demon chooses to make the reverse gravity only 10ft high the PCs caught in it will be floating and oscillating there.

TL;DR
Will the DC be 10+spell level or 15+Spell level or no concentration check at all?

[Skylancer4]

Unless the Reverse was cast in the middle of a spell being cast I don't see why it would cause issues with spell casting. The "violent motion" would make sense if they started "falling" mid cast, but once they "fell" and were suspended I would look at levitate for how to handle it (and it doesn't mention spell failure or concentration checks for spell casting, just penalties for attacking and being off balance).

[Just a Guess]

With levitate you have a certain degree of control, with reverse gravity you don't. You are being moved (the oscillating). I might use the levitate rules as you suggested but I'm not convinced they are enough.

[Trekkie90909]

Reverse Gravity wrote:

This spell reverses gravity in an area, causing unattached objects and creatures in the area to fall upward and reach the top of the area in 1 round. If a solid object (such as a ceiling) is encountered in this fall, falling objects and creatures strike it in the same manner as they would during a normal downward fall. If an object or creature reaches the top of the area without striking anything, it remains there, oscillating slightly, until the spell ends. At the end of the spell duration, affected objects and creatures fall downward.

Provided it has something to hold onto, a creature caught in the area can attempt a Reflex save to secure itself when the spell strikes. Creatures who can fly or levitate can keep themselves from falling.

There are many variables to this question:

Assuming that the caster cannot fly or levitate/is not currently benefiting from them. Further assuming there is nothing to grab onto:

1) Is the caster mid-cast, or are they trying to cast after gravity has already reversed? Lets call mid-cast 1a, and already reversed 1b. If 1a is true then treat it like falling; violent motion DC 15+SL. if 1b is true then it depends which of 2 or 3 is true:

2) In an enclosed space: Any concentration check from 1 is applicable, and if 1a is true then they also need to make a check once they take falling damage. If 1b then they are standing stable on the ceiling and require no check from reverse gravity.

3) In an open space: Any concentration check from 1 is applicable. If 1a is true then a further check may be required if the spell requires an additional round to finish. Otherwise 1a does not require an additional check. If 1b is true then vigorous motion rules apply and the DC is 10 + SL.

Still Assuming the caster cannot/is not flying/levitating, but now there is something to grab onto (and they try to do so):

4) 1a is true: DC is reflex DC + SL. 1b is true: arguable possibly oscillating (in which cast 10 + SL), possibly not (no check). On a successful reflex save they possibly can't use spells which require somatic components as their hands are wrapped around the pillar. On a failed save go to 1).

Assuming now that they can/are flying but not levitating, and do not try to anchor themselves.

5) First they make a fly check to remain stable in the air. If they fail then treat as if they are not flying and start at 1) or 4). Assuming 1a is true they would need to make an additional concentration check DC=Fly DC + SL. If they succeed then if 1a or 1b is true they need to make a concentration check DC = fly DC + SL.

Assuming levitating but not flying:

6) No check is required for the first spell; if the spell involves an attack then the caster becomes increasingly unstable and requires additional checks (starting at DC 10 + SL, and increasing until they spend a turn regaining their balance)

Assuming Levitating and Flying:

7) Ugh. Just kill them please.

[Diego Rossi]

"Oscillating slightly" is very, very far away from violent motion.
At worst it is equivalent to being on the deck of a ship with a sea force of 1. Or being in a hammock.
I wouldn't require a concentration check for that.

Look the definition of Vigorous motion:

PRD wrote:

Vigorous Motion: If you are riding on a moving mount, taking a bouncy ride in a wagon, on a small boat in rough water, belowdecks in a storm-tossed ship, or simply being jostled in a similar fashion, you must make a concentration check (DC 10 + the level of the spell you're casting) or lose the spell.

There is a big difference between "oscillating slightly" and "taking a bouncy ride in a wagon" or any of the other examples.

[Skylancer4]

Just a Guess wrote:

With levitate you have a certain degree of control, with reverse gravity you don't. You are being moved (the oscillating). I might use the levitate rules as you suggested but I'm not convinced they are enough.

They are the closest in game mechanic we have to what you are asking, whether or not you feel they are enough for a question and situation that isn't explicitly defined by the rules is fairly irrelevant. They are what we actually do have.

It isn't vigourous motion once the effect is resolved.

It isn't an attack or ongoing damage.

It is the result of a spell altering the environment, which is different from "under the effect of a non damaging spell".

As it doesn't fit any of the explicitly listed scenarios for concentration checks, we can start looking for the similarly stated mechanical possibilities to use. The closest we have is Levitate. A situation where you are essentially placed at a point with little control of your movement. If you are a fighter being levitated by the party wizard, you have no control over your movement. You get anchored at whatever height they choose.

That is the closest in game mechanic I can find in the CRB to offer you for your question. It is what we do have in absence of actual rules regarding the scenario.