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Could a Huge or bigger creature use antimagic field?

The antimagic field spell description says:

Range: Self (10-foot radius sphere)

A 10-foot-radius invisible sphere of antimagic surrounds you. [...] Until the spell ends, the sphere moves with you, centered on you.

The Variant: Dragons as Innate Spellcasters box under the Dragons entry in the MM (p. 86) allows a Dragon to cast spells of a level 1/3 their CR:

Dragons are innately magical creatures that can master a few spells as they age, using this variant.

A young or older dragon can innately cast a number of spells equal to its Charisma modifier. Each [...] spell's level can be no higher than one-third the dragon's challenge rating (rounded down).

So a CR24 Ancient Dragon can cast 8th-level spells and be Gargantuan.

A Gargantuan dragon's "self" is larger than 10 feet. So is there a ten-foot-radius sphere that pops up deep inside the dragon's body? Or does it surround the dragon's body to an area of 10 feet around it?

I'd tend to say the latter, but I'm not sure how to adjudicate that. The two parts of the spell description are mutually exclusive and neither is more or less specific than the other.

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    \$\begingroup\$ Bigger than 20 feet, I suppose? 10 ft is a radius. \$\endgroup\$ – enkryptor Oct 31 '18 at 22:17
  • \$\begingroup\$ I don't have my monster manual in front of me. What huge or larger creature(s) possess the ability to cast AMS? \$\endgroup\$ – Rykara Oct 31 '18 at 22:19
  • \$\begingroup\$ @rpeinhardt the variant rules for Dragon spellcaster could gain access and there are ways to increase creature size. I don't think the absence of huge spellcaster would even be relevant. \$\endgroup\$ – David Coffron Oct 31 '18 at 22:30
  • \$\begingroup\$ @enkryptor Good point, edited the title to address this @ rpeinhardt I addressed this question in my edit. Variant spellcasting rules for Dragons in MM sidebar. \$\endgroup\$ – user47897 Oct 31 '18 at 22:40
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The way I've always interpreted radius on spells is that it extends from the exterior of the caster, rather then the interior of the caster.

I believe this justifies the "Until the spell ends, the sphere moves with you, centered on you." condition, assuming the centered on you excludes the inside of the caster's body.

This just means that the caster is the origin, this doesn't detract from the units of measurement in any other direction.

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    \$\begingroup\$ +1 Whether or not that is RAI or RAW, that is a really swell solution! \$\endgroup\$ – Lexible Nov 1 '18 at 1:24
  • \$\begingroup\$ I like this answer and it is often how I think of it. Probably not RAW to be fair, but I think still valid and adds to the fact that larger creatures are often meant to be challenging. \$\endgroup\$ – SeriousBri Nov 1 '18 at 9:15
  • \$\begingroup\$ This is correct. A 5' radius spell centered on the caster, for example, extends 5' out in all directions; the effect actually has a 15' diameter (and thus a 7.5' radius) if you just look at the effect area itself. \$\endgroup\$ – Xanthir Nov 4 '18 at 16:30
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    \$\begingroup\$ Excellent Answer. This is also backed up by a twitter conversation Jeremy Crawford had. (See the replies to clarify his meaning). \$\endgroup\$ – Gandalfmeansme May 7 '19 at 19:20
  • \$\begingroup\$ I have to ask though: if the spell extends 10 feet out from the exterior of the caster's body (or its space if you find that more convenient), how is its area of effect still spherical? It seems to me that this would only happen if the craster's body (or space) was spherical to begin with. Otherwise (if we go from its space), it would more resemble a square with curved corners, wouldn't it? And doesn't that cause a problem with the fact that the range is "Self(10 foot radius sphere)"? \$\endgroup\$ – Gandalfmeansme May 8 '19 at 18:22
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The rules on Areas of Effect state (original emphasis):

Every area of effect has a point of Origin, a location from which the spell’s energy erupts. The rules for each shape specify how you position its point of Origin. Typically, a point of Origin is a point in space, but some spells have an area whose Origin is a creature or an object.

This indicates that the origin of an effect is a "point" - in space or in or on a creature.

Turning to the Dungeon Master's Guide (p. 250) under "Using Miniatures", it says:

Choose an intersection of squares or hexes as the point of origin of an area of effect, then follow its rules as normal.

Your dragon can choose any corner of any of the 16 squares it occupies as the point of Origin - the antimagic field extends 10-feet from there. When the dragon moves, it chooses a new point of origin within its new 16 squares.

The effect of this on the dragon is:

  • The dragon cannot be targeted by magic as it is "a creature ... in the Sphere"
  • The dragon can be affected by areas of effect that cover a square of the dragon the antimagic field doesn't. Noting that if the dragon chose the middle square all of it would be within the sphere.
  • Spells and magical effects on the dragon are suppressed.
  • Magic weapons used by or against the dragon are non-magic unless they are missile weapons fired or thrown by the dragon.
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    \$\begingroup\$ You might want to link to DNDBeyond or another SRD rules source instead (for the "areas of effect" rules); Roll20's compendium link formatting causes words to be incorrectly capitalized, though they aren't in the source text. (I guess the incorrect capitalization also occurs in the inline quote in your first bullet point.) Relevant meta: How to handle incorrect quotations from 3rd party sources \$\endgroup\$ – V2Blast♦ Nov 1 '18 at 2:18
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    \$\begingroup\$ @V2Blast Not capitalizing every game term is the worst formatting decision the game designers made in my opinion. Clarity is more important for me than being consistent with something bad. \$\endgroup\$ – András Nov 2 '18 at 8:28
  • \$\begingroup\$ @András: It's generally pretty obvious which terms are game terms and which ones aren't. The confusion generally only occurs when there is a game term that refers to two different things, depending on whether or not it's capitalized (e.g. attacks and Attack actions) - but that problem exists independent of every game term not being capitalized. \$\endgroup\$ – V2Blast♦ Nov 2 '18 at 22:41
  • \$\begingroup\$ @V2Blast, if you are restrained, you can't use somatic components. If you are Restrained, you can cast normally, just have disadvantage on attack rolls. Every game term that you can find in the basic dictionary is a source of problems, for example 6 hours can feel like a long rest, but for a Long Rest, you need 8, and so on. \$\endgroup\$ – András Nov 2 '18 at 22:56
  • \$\begingroup\$ Conditions (e.g. restrained) aren't capitalized in the game rules. For instance, see the description of watery sphere. And the term never gets used in the game rules except when it refers to the condition, so it's never really confusing... Your source of confusion seems to indicate people using game terms to refer to things other than what the game defines them as, which is really a self-created problem. Distinguishing game terms by capitalizing them wouldn't make that less confusing. \$\endgroup\$ – V2Blast♦ Nov 2 '18 at 23:52

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