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?
Best Answer
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:
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.