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If there is a 20-foot by 10-foot passage, can two adjacent gelatinous cubes (or similarly large ooze) block line of effect? If the cleric casts prayer, are enemies behind the cubes protected from the spell?

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Likely yes, but there is some ambiguity

The size of a normal gelatinous cube is 10 feet to a side, and as a large creature, it occupies a square 10 feet to a side. They are unusual in that respect, because other than most other creatures, they fill up the enitre space they occupy in combat. So the two cubes (unless they were starving or smaller than normal) would entirely fill the corridor.

Line of Effect states:

A line of effect is a straight, unblocked path that indicates what a spell can affect. A line of effect is canceled by a solid barrier. It's like line of sight for ranged weapons, except that it's not blocked by fog, darkness, and other factors that limit normal sight. (...)

A burst, cone, cylinder, or emanation spell affects only an area, creature, or object to which it has line of effect from its origin (a spherical burst's center point, a coneshaped burst's starting point, a cylinder's circle, or an emanation's point of origin).

The unblocked path that is requried is cancelled by a solid barrier. The question then becomes if the cubes count as a solid barrier. If they do, they will block line of effect. As prayer has a burst effect, so they also would block enemies behind them from being affected by the spell.

Based on this accepted answer, the bodies of creatures count as solid barriers, even when they are not made from hard solids like a stone wall. However, the logic in the answer is arguing from total cover for someone swallowed by a creature, and total cover in turn is granted by a solid barrier, so we are back to the question if a creature's body counts as a solid barrier.

I don't think there is an official definition of the term or statement about it in the rules.

  • Creatures count as soft cover, which is a kind of cover alongside half, partial, total and improved cover, which could suggest they count as obstacles, just like harder substances.
  • Creatures are clearly physical obstacles in the way, much more so than the fog or darkness given as counterexamples.
  • At the same time, Gelatinuous cubes in particular are not really solid, as defined in the dictionary, that is "firm or stable in shape; not liquid or fluid". They are fluid. They can engulf you. If you are engulfed but not paralyzed and manage to free yourself from being pinned, you can move out of them.

As there is no clear-cut definition, the DM will have to make a final call on this.

The rules have this exception for blocking line of effect:

An otherwise solid barrier with a hole of at least 1 square foot through it does not block a spell's line of effect. Such an opening means that the 5-foot length of wall containing the hole is no longer considered a barrier for purposes of a spell's line of effect.

That's a pretty big hole that is needed to enable line of effect. A smaller one, say fist-sized, would not count, even though you could cleanly look or shoot through it. Based on this, on the first two bullets in the list above, and on the accepted answer for swallowed creatures, I would rule that a soft barrier like a Gelatineous Cube that fully fills the space also blocks line of effect.

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    \$\begingroup\$ I didn't answer this question since I couldn't decide if a gelatinous cube counted as a "solid barrier" since it's, y'know, a gelatinous barrier, but I do think this is as accurate as you can get RAW. \$\endgroup\$
    – Shivers
    Apr 29, 2023 at 10:52
  • \$\begingroup\$ I believe "solid" in this case means not full of holes or the like, not that it needs to be tough. Thus it seems to me obvious that the gelatinous cube blocks line of effect. \$\endgroup\$ Apr 29, 2023 at 22:17

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