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This Q&A in regards to emanations suggests that extending some of them from a creature's space rather than a grid intersection is a good way to handle those effects that emanate from a creature rather than from a point in space. I like the answer there.

Preamble: My low-level wizard character is about to embark on a raid on a beholder's lair, and wants to have most of the party invisible. His plan includes casting invisibility sphere (PH 245) on the party and then have them ahead while he's a short way behind in an antimagic field (PH 200) which will be both refuge and trap for the beast. Due to his inability to see in darkness, the need to not alert the beholder to their presence too early by carrying a working light source, and the silence (PH 279) spell he intends to have the rest of the party under, he wants to have a rope extend from the party, most of whom have 60 foot darkvision (which he doesn't possess) so that they can all gauge their distance from each other, as well as send very basic signals.

I got the idea that making the rope invisible wouldn't hurt, and this question sprang to mind:

When cast on a very large or changeable object (like a rope), how do spells like invisibility sphere and silence operate? The invisibility sphere and silence spells say that they can be cast on objects which then emanate the spell effect from themselves. Both effects are also explicitly mobile with the recipient.

Would the effect persist along the entire rope, permitting humorously large groups of silent, invisible creatures who were all within 10 feet of an uncoiled rope (less than 100 lbs of rope in order to stay within the invisibility spell's limits)?

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You need to pick a point of origin for the emanation

The Q&A you are linking is discussing how they handle the fact that it is not clear what intersection to use to represent the point of origin. It shows a method that in practical play seems to work smoothly, but it is explicit that this is not an official rule.

The official rules state (PHB, Aiming a Spell):

Regardless of the shape of the area, you select the point where the spell originates

Burst, Emanation, or Spread: Most spells that affect an area function as a burst, an emanation, or a spread. In each case, you select the spell’s point of origin and measure its effect from that point.

This is very clear and explicit that you always have to chose a point from which the radius for the spell is counted.

Therefore, it will not be possible to have a 50-feet long invisibility sphere extend around the eponymous rope. You need to pick one point along the rope.

I believe the confusion here comes from following the practical but inoffical resolution recipe to an extreme where it stops to be practical and instead produces unexpected results. Just return to the staid rules as written, and the problem dissolves.

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  • \$\begingroup\$ Are you suggesting that casting invisibility, invisibility sphere, or silence on an object should be treated differently than casting them on a creature? I mention invisibility since invisibility sphere inherits from it. Yes the ruling there is a 'house rule', but is largely a necessary one, because the rules don't otherwise function usefully... \$\endgroup\$
    – Chemus
    Mar 11, 2022 at 1:04
  • \$\begingroup\$ I'm not suggesting that. The rules apply to objects and creatures. If you want my opinion, I'm more in the Pirates of the Carribean camp of "they're more like guidelines" and in cases where the rules are not working well, would make a house rule - if all of you are happy to extend the effect, have fun and do it. This site however encourages factual answers with references. \$\endgroup\$ Mar 11, 2022 at 6:13
  • \$\begingroup\$ I understood that, and you are correct. I'm kind of 'security minded'; I look for holes and handles. Sometimes it's useful. I observed that making the spells easily functional for larger creatures also gave the spells... potential... when used on objects. \$\endgroup\$
    – Chemus
    Mar 11, 2022 at 6:22
  • \$\begingroup\$ @Chemus in this case, rules function relatively well both with or without mentioned houserule. Little difference in case of an object, probably, as rules suggest DMs to split large objects into sections. \$\endgroup\$ Mar 11, 2022 at 13:39
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This does not work as you had hoped.

Although invisibility sphere may be cast on an object, it does not affect the object. Invisibility sphere specifically operates on creatures within a radius. If it was worded "functions like invisibility, except that this spell also confers invisibility upon all creatures within 10 feet of the recipient", then you could cast invisibility on the rope, choose the grid mark for the emanation to be at the point group, and the entirely of the rope would remain invisible. However, the language of the spell instead dictates that it exclusively applies invisibility to creatures, so any invisibility granted to objects would have to be through possession of those creatures.

If a creature was holding the rope, then the rope would be invisible out to 10 ft from the creature, at which point the remainder of the rope would become visible, as the invisibility spell operates.

Additionally, even if you were to cast invisibility (not invisibility sphere) directly on the rope itself, any section of the rope within the area of the anti-magic field would be visible per normal.

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  • \$\begingroup\$ Area: 10-ft.-radius emanation around the creature or object touched [...] functions like invisibility, except that this spell confers invisibility upon all creatures within 10 feet of the recipient. The center of the effect is mobile with the recipient - invisibility sphere So, the object touched doesn't become invisible by your reading? \$\endgroup\$
    – Chemus
    Mar 11, 2022 at 22:35
  • \$\begingroup\$ @chemus That is correct. The spell does not imply that the target recipient is made invisible. It only grants invisibility to creatures within the area, and that area is simply based on the target recipient. \$\endgroup\$ Mar 11, 2022 at 23:29
  • \$\begingroup\$ If it said "also confers invisibility" instead if how it is phrased, then the target would receive invisibility, too \$\endgroup\$ Mar 11, 2022 at 23:31
  • \$\begingroup\$ So the 'works like invisibility, except' doesn't inherit making the target of the spell invisible from invisibility? \$\endgroup\$
    – Chemus
    Mar 11, 2022 at 23:50
  • \$\begingroup\$ 'works like invisibility' except that it confers invisibility to creatures within a 10' radius of the target.... \$\endgroup\$ Mar 12, 2022 at 8:39

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