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We're running Dragon Heist and the party is facing the Xanathar himself in a battle. I as the DM have decided to make this encounter a bit more fun by letting some of the spectators to play as NPCs (including the Beholder itself)

One of the high-level NPC wizards, who was helping the party in this fight, decided to cast Forcecage on the scary Beholder.

But the guy playing the Beholder (ex DM) said that this tactic wouldn't work with Xanathar, because:

  1. The wizard would not be able to "complete" the Forcecage spell in the first place, because the Beholder had his Antimagic Cone active, not letting the spell to complete.
  2. Even if Antimagic Cone was turned off at the moment of casting the Forcecage spell, the Beholder will just turn it on and will "cut a hole" in the cage so that he would be able to escape.
  3. Even if Beholder will be trapped waiting for the Disintegrate Ray, he still would be able to use his annoying Antimagic Cone as before, because the Antimagic Cone would easily "cut through" the cage and beyond.
  4. Also, the deadly Beholder Rays will work through the Forcecage because it's said that Forcecage blocks spells, but Beholder's rays are NOT "spells".
  5. He also mentioned that Antimagic Field is level 8 and Forcecage is level 7, Wall of Force is even lower, level 5.

I've searched for an answer on this website, but the closest what I've found is this, saying that an already "existing" Forcecage probably will not be affected by the Antimagic Field. So it's not exactly the Beholder vs Forcecage spell... Also that question was answered for DnD 3.5e...

I've shown this link to the Beholder's player but he says that the Forcecage description says "a forcecage resists dispel magic", but the Beholder's Antimagic Cone is not "Dispell Magic", so wherever the Beholder points with his Antimagic Cone, all Forcecages and Walls of Force WILL be supressed. I really don't see what's wrong with this logic 🤷.

Because of that, he said that Forcecage and Wall of Force are completely useless in an encounter with a Beholder - he'll be just suppressing them. Are they, really?

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    \$\begingroup\$ Ah, so you have a dungeon master running the game, and a player using the NPC beholder instead of a traditional player character? \$\endgroup\$ Nov 7, 2022 at 0:03
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    \$\begingroup\$ @ThomasMarkov yes. Actually, it turned out to be quite fun! We'll probably use this trick even more, if someone want's to enjoy some DnD but is not ready to commit for a full campaign. Al least for such epic encounters like this one. \$\endgroup\$
    – Lisa
    Nov 7, 2022 at 0:07

2 Answers 2

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Antimagic Cone. The beholder's central eye creates an area of antimagic, as in the anti magic field spell, in a 150-foot cone.

Monster Manual, p.28

Areas of Magic. The area of another spell or magical effect, such as fireball, can’t extend into the sphere. the sphere overlaps an area of magic, the part of the area that is covered by the sphere is suppressed. for example, the flames created by a wall of fire are suppressed within the sphere, creating a gap in the wall the overlap is large enough.

Player's Handbook p.214

... blocking any spells cast into or out from the area.

Player's Handbook p.243

So:

  1. The Forcecage spell will compete as part of it will be outside the antimagic cone and it is not targeted at a creature or object inside the cone. Of course, if the caster is within the cone, they can't cast any spell including Forcecage. However, the part that is within the cone is suppressed i.e. there is a hole in the cage. However, this does not mean the beholder can escape - see below.
  2. Correct but see above and also below.
  3. Yes.
  4. Correct.
  5. Yes, he has correctly identified the levels of those spells - no idea what the relevance is.

Now, even though there is a hole in the cage in front of the beholder, the beholder can't actually get out because as it approaches the cage, the area of the cage suppressed gets smaller because that's how cones work. It can't get out because the hole gets too small for it to fit through.

So, while Forcecage is limited against a beholder it can effectively immobilise it which might be useful - especially if you want to run away. Which is always a good strategy for surviving an encounter with a beholder.

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    \$\begingroup\$ At least it will immobilize the Beholder for a short while. They still can hit the cage with the Disintegrate Ray at some point to make it disappear. I think the levels argument is that higher level effects should trump lower level ones, which is a useful idea, even though not a general rule outside special cases like darkness, globe of invulnerability or counterspell. (+1) \$\endgroup\$ Nov 7, 2022 at 4:46
  • \$\begingroup\$ @GroodytheHobgoblin I would expand that list with Globe of invulnerablility as it is very similar to anti magic field when it comes to cutting area of effect spells. \$\endgroup\$
    – findusl
    Nov 27, 2022 at 14:20
  • \$\begingroup\$ When the spell is initially cast, the beholder's antimagic cone will be blocking part of it. Will that blocked part of the spell be permanently missing (thus allowing a possible escape) or will it appear once the antimagic cone is removed? AMF does say "Within the sphere, spells can't be cast", which suggests that it should suppress the initial casting of the cage within its area. (Or is this clause just saying that spells can't be cast while the caster is in the area?) \$\endgroup\$ Nov 27, 2022 at 14:53
  • \$\begingroup\$ @RyanC.Thompson No, you need to read the next paragraph where it explains that “can’t be cast” actually means “suppressed”. The spell can be cast but it’s effects don’t happen while it’s in the cone. \$\endgroup\$
    – Dale M
    Nov 27, 2022 at 20:34
  • \$\begingroup\$ Made an answer based on this, but since according to RAW a cone can include its point of origin at its creator's discretion, if you put the origin of the cone in the beholder's space and include it in its own cone, the shrinking hole issue is resolved by squeezing for the last five feet of movement, and the beholder can escape. \$\endgroup\$ Nov 28, 2022 at 1:05
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By a RAW reading, your (ex)DM appears to be correct.

A cone extends in a direction you choose from its point of origin. A cone's width at a given point along its length is equal to that point's distance from the point of origin. A cone's area of effect specifies its maximum length.

A cone's point of origin is not included in the cone's area of effect, unless you decide otherwise.

Emphasis mine

let's drop a little grid for reference really quick.

So our beholder is occupying squares (12,13,22,23) as a large creature. Our force cage or wall of force in a 10 foot cube would occupy the outside perimeter of these same squares.

If beholder so chooses, the beholder's cone originates at the intersection of (22,23,32,33), which is an intersection in its space, with orientation chosen specifically and only at the start of its turn as "directly north." The cone progresses to 5 feet in width by the intersection (12,13,22,23), and 10 feet in width by the intersection of (2,3,12,13) creating a hole wide enough in the cage effect for the beholder to enter, and then finish exiting by squeezing through the 5 foot section of hole that remains when he's moved 5 feet north, which addresses the concern that as it moves north the hole would shrink.

This interaction occurs the same way regardless of which spell we're talking about, and also if you make the forcecage 20 feet on a side, because the beholder can still make a hole large enough to get through. While neither effect is affected by dispel magic, both effects are clearly ongoing spells that would have their area suppressed in an antimagic field.

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