Is the point of origin for a persistant AoE spell relevant after casting?

Question

When a spell is cast, it is understood that there must be an obstruction-free line of effect from the point of origin of an area of effect spell for it to affect a location (PHB 204):

A spell’s effect expands in straight lines from the point of origin. If no unblocked straight line extends from the point of origin to a location within the area of effect, that location isn’t included in the spell’s area.

For a persistent area of effect spell, is this point of origin only relevant on casting of the spell, or is the spell constantly emanating from that point of origin for the entire duration?

Example

Entangle spell description (PHB 238):

Grasping weeds and vines sprout from the ground in a 20-foot square starting from a point within range. For the duration, these plants turn the ground in the area into difficult terrain.

A creature in the area when you cast the spell must succeed on a Strength saving throw or be restrained by the entangling plants until the spell ends. A creature restrained by the plants can use its action to make a Strength check against your spell save DC. On a success, it frees itself.

When the spell ends, the conjured plants wilt away.

For example, the entangle spell (text above), creates a 20-ft square area from the point of origin. Say entangle is cast in the middle of an empty room with no obstructions, then afterwards, a wizard casts wall of stone to cut the affected area in half. Do the weeds and vines that are no longer in line-of-effect from the point of origin end immediately, or do they continue to last for the duration?

Could I throw a blanket over the point of origin to essentially block the entire spell?

Answer 1

It depends on the spell.

In general, my reading is that the spell effect comes into existence in the specified area, and any subsequent alterations to the area don't change that.

But, it depends on the spell description. Some spells are explicit that the effect in the area is ongoing and directional, in which case an appropriate piece of cover should exclude that part of the area.

For example, gust of wind creates "a line of strong wind 60 feet long and 10 feet wide" that "blasts from you". It seems obvious that if a wizard throws a wall of stone into the path of the wind, it should stop the wind. You wouldn't have the wind blasting from the enemy caster, then somehow going through the wall to continue to fill the previously defined area.

Similarly, a moonbeam spell "shines down", and I would certainly expect that tossing an opaque horizontal surface in the way would create a shadow that's protected from the beam.

But on the other hand, there's no reason to think that putting a wall through a patch of entangle or grease stops the plants from being big and enchanted or the floor from being slippery.

I think the area continues to be the area of the spell unless the spell specifically calls out or implies that something is being continually emitted from the origin point. I know that isn't a great answer, but I don't think there's a single rule that makes sense in all cases. Certainly "put a bucket over the origin" would be absurd and potentially trivializing for a lot of magic.

There's a notable "exception that proves the rule" in darkness and daylight, which both have similar text that specifically allows bucketing the spell:

If the point you choose is on an object [...] the darkness emanates from the object and moves with it. Completely covering the source of the darkness with an opaque object, such as a bowl or a helm, blocks the darkness.

The fact that this is called out in specific is evidence that spells are not expected to work that way by default. Furthermore, darkness and daylight themselves can't be blocked by a container if they're targeted at a point in space rather than a physical object, since the "covering" text is only given in the section about having an object that emanates light or darkness.

Answer 2

Careful spell reading

In some cases, AoE spells have effects that continually radiate from their point of origin. In some cases, they do not. Ideally, paying careful attention to the language of the spell can tell us what is happening.

In the case of Entangle:

Grasping weeds and vines sprout from the ground in a 20-foot square starting from a point within range. For the duration, these plants turn the ground in the area into difficult terrain...When the spell ends, the conjured plants wilt away.

(1) At the time of casting, the point chosen acts as the origin of the spell effect.
(2) The effect causes weeds and vines to sprout within the AoE at that point in time
(3) Once the weeds and vines have sprouted, their effect lasts for the duration of the spell
(4) The effect ends when the spell ends

Conclusion: Covering the point of origin does nothing after the spell is cast; affecting a square in which the effect persists (such as with wall of stone) affects only those squares.

Example case: Darkness cast at a point:

Magical darkness spreads from a point you choose within range to fill a 15-foot-radius sphere for the duration. The darkness spreads around corners. A creature with darkvision can't see through this darkness, and nonmagical light can't illuminate it.

(1) At the time of casting, the point chosen acts as the origin of the spell effect.
(2) The effect causes darkness within the AoE at that point in time
(3) Once the darkness has spread, its effect lasts for the duration of the spell
(4) The effect ends when the spell ends

Conclusion: Covering the point of origin does nothing after the spell is cast

Example case: Darkness cast on an object:

If the point you choose is on an object you are holding or one that isn't being worn or carried, the darkness emanates from the object and moves with it. Completely covering the source of the darkness with an opaque object, such as a bowl or a helm, blocks the darkness.

(1) At the time of casting, the point chosen acts as the origin of the spell effect.
(2) The effect causes darkness within the AoE at that point in time and continues to emanate from that point

Conclusion: Covering the point of origin blocks the effect of the spell

The spell itself, implicitly or explicitly, should describe whether the effect continually flows from the origin point or does so only once and then resides in the Area

Answer 3

If the spell does not state what happens, it's a case of ruling

The rule for area of effect:

A spell's effect expands in straight lines from the point of origin. If no unblocked straight line extends from the point of origin to a location within the area of effect, that location isn't included in the spell's area. To block one of these imaginary lines, an obstruction must provide total cover. (p. 204 PHB).

Simple cases

Some spells with persistent areas explicitly state they are emanating from a given point (for example, darkness, daylight) and will be blocked by cover, so there is no question. Others state they emanate from a given point or area during the spell's duration (for example, moonbeam, gust of wind, wall of fire). Their effects will be blocked by cover too.

What remains is resolution for how this works when the spell does not say anything explicitly, like with entangle, or stinking cloud:

Arguments for area of effect being checked continually

• The rule in the PHB does not contain any language that would limit it to the time of casting

• Existing highly upvoted precedent answers like that for Wall of Light indicate that it is checked continuously

• The ongoing effects are not natural poison gas, vines etc., they are magical ephemera, and therefore follow the laws of magic, not the laws of normal physical objects

Arguments for area of effect only being checked during casting

• Continuous checking leads to counterintuitive results to what one would expect for mundane effects -- why would the vines vanish if you place a wall through them?

• Circumstantial support from the examples given in the area of effect section in the PHB. Both examples cited (burning hands and cone of cold) are instantaneous, and it would have been easy to replace one of them with an example that creates a continuous effect like entangle.

• Circumstantial support from the fact that some spells with ongoing effects have explicit language about blocking line of effect during their duration, so this should not be assumed as default

I think neither side can present a strong enough argument to conclusively settle the issue, so until we get Sage Advice clarification, these edge cases will need adjudication by the DM.