DND 5E Spells – How Does Beholder’s Antimagic Cone Interact with Forcecage and Wall of Force?

An existing forcecage spell's effect is probably unaffected by an incoming antimagic field spell's effect

A spell that creates multiple walls of force—like the the 4th-level Sor/Wiz spell force chest [evoc] (Spell Compendium 97) or the 7th-level Sor/Wiz spell forcecage [evoc] (PH 233)—requires using the rules for the 5th-level Sor/Wiz spell wall of force [evoc] (PH 298-9), and those include how the wall of force spell's effect interacts with the effect of the 6th-level Sor/Wiz spell antimagic field [abjur] (PH 200), this despite the antimagic field spell's description specifying only the spell wall of force.

The spell forcecage says

Like a wall of force spell, a forcecage resists dispel magic, but it is vulnerable to a disintegrate spell, and it can be destroyed by a sphere of annihilation or a rod of cancellation.

Compare that with the spell wall of force, which says that it's effect

is unaffected by most spells, including dispel magic. However, disintegrate immediately destroys it, as does a rod of cancellation, a sphere of annihilation, or a Mordenkainen’s disjunction spell.

The reader must refer to the spell wall of force to learn more about the spell forcecage (otherwise, for example, the phrase "vulnerable to a disintegrate spell" is meaningless). And, if trying to determine how, for example, the antimagic field spell's effect interacts with the force cage spell's effect, the reader'll end up back at the spell wall of force anyway.

It would take an extremely strict reading of the spell antimagic field—followed by a personal definition of what a forcecage spell's effect's now-mysterious walls of force were actually made of—to have an existing forcecage spell's effect suppressed by an antimagic field spell's effect.

Walls of force usually block line of effect but an antimagic field spell's effect does not

Normally, the antimagic field spell's effect just doesn't care about a magical effect that would block its line of effect: the antimagic field spell's effect causes the existing magical effect to be suppressed. But, like the spell antimagic field says—and the Rules Compendium confirms—, "A wall of force… isn’t affected by antimagic" (11).

Thus even an existing forcecage spell's barred cage effect allows spells to be cast while those within would be, if uncaged, in an antimagic field spell's effect, although usually not beyond the barred cage effect's walls. While an "otherwise solid barrier with a hole of at least 1 square foot through it does not block a spell’s line of effect" (PH 176), the forcecage spell's barred cage effect has only "half-inch wide [bars] with half-inch gaps between them."

However, casting the spell forcecage while within an existing antimagic field spell's effect and picking as the forcecage spell's effect's point of origin a crosshairs within the antimagic field spell's effect means the forcecage spell's effect is suppressed while the antimagic field spell's effect remains. (However, picking for the forcecage spell's effect's point of origin a crosshairs that's outside the antimagic field spell's effect seems to mean the forcecage spell's effect functions normally. The antimagic field spell's effect doesn't block line of effect (RC 11)—an apparent reversal of how the spell antimagic field functioned according to the Dungeons and Dragons, Third Edition FAQ—, but opinions on exactly how this works vary widely.)

Excellent Idea! (MAYBE there's a loophole)

First of all, I'm impressed by this strategy! It's a great idea, and turns the tables on the Beholder extremely well. You might have some trouble targeting the Beholder from within the cloud yourself, but characters normally know where other creatures are during combat (the fact that the Beholder hovers may complicate this, but probably not unless the Beholder takes the Hide action).

There is a small potential loophole I noticed, but it's not ironclad (more open to interpretation).

Legendary Action & Lair Actions

The Beholder can use its eye rays with its Action during its turn, but must choose targets it can see. However, it can also use its eye rays in two other ways: one is with a Legendary action at the end of another creature's turn. This legendary action is described as follows:

Eye Ray. The beholder uses one random eye ray.

It can also use lair actions, one of which is the following (MM, p. 27, bold added):

Walls within 120 feet of the beholder sprout grasping appendages until initiative count 20 on the round after next. Each creature of the beholder's choice that starts its turn within 10 feet of such a wall must succeed on a DC 15 Dexterity saving throw or be grappled.

Note that neither of these descriptions requires the Beholder to see its chosen targets.

It's likely that the definition of the eye rays given in the Actions section of its description is meant to apply to the other (Legendary) rays as well. But this is ambiguously worded, and open to interpretation. Clearly, some parts of the description of the eye rays must apply (or else there would be no limit to the range of these Eye Rays, for example): exactly how much applies, though, is ultimately up to the DM.

Even if its lair and Legendary actions do not require sight, note that your strategy would pretty much halve the amount of opportunities that the Beholder has to target creatures directly (as opposed to lifting a heavy object above a character while they are in the Antimagic Cone, and dropping it). This makes this strategy an excellent idea in any case.