Can you attack a creature behind a Wall of Sand?

Question

Wall of Sand's description reads as follows (emphasis mine):

You conjure up a wall of swirling sand on the ground at a point you can see within range. You can make the wall up to 30 feet long, 10 feet high, and 10 feet thick, and it vanishes when the spell ends. It blocks line of sight but not movement. A creature is blinded while in the wall’s space and must spend 3 feet of movement for every 1 foot it moves there.

To me, it's unclear whether or not the bolded clause provides total cover to a creature behind the wall, or if the creature is simply heavily obscured. If you and another creature are on the opposite sides of a Wall of Sand, can you perform a ranged weapon or spell attack against the creature?

Answer 1

Yes, you can attack through a Wall of Sand.

A target has total cover if it is completely concealed by an obstacle. —Player's Handbook, pg. 196

"Obstacle" is not a game term, but all the examples given are of solid objects (walls, trees, creatures, and furniture), which agrees with the dictionary definition "a thing that blocks one's way" (Lexico).

As you've noted, Wall of Sand does not block movement, so it is not an obstacle, and therefore does not provide total cover.

As a side note, the attack would have disadvantage, since

When you attack a target that you can't see, you have disadvantage on the attack roll. —Player's Handbook, pg. 194

except that the target can't see you either (assuming it doesn't have blindsight or similar abilities), and

When a creature can't see you, you have advantage on attack rolls against it. —Player's Handbook, pg. 195

so the advantage and disadvantage would cancel out.

Answer 2

Some attacks will be negated, others will not

Any attack that requires the attacker to see the target (the cantrip sacred flame is an example) cannot attack the target. If the line of sight is blocked, so too is the ability to see the target, which would render the target either wholly obscured or behind full cover - that would depend somewhat on the situation. A few examples include:

• Acid Splash: one or two creatures you can see within range
• Chain Lightning: You create a bolt of lightning that arcs toward a target of your choice that you can see within range
• Disintegrate: A thin green ray springs from your pointing finger to a target that you can see within range.
• Sacred Flame: Flame-like radiance descends on a creature that you can see within range.

Other attacks where you can't see the target may be successful, or they may not.

Unseen Attackers and Targets
When you attack a target that you can’t see, you have disadvantage on the attack roll. This is true whether you’re guessing the target’s location or you’re targeting a creature you can hear but not see. If the target isn’t in the location you targeted, you automatically miss, but the DM typically just says that the attack missed, not whether you guessed the target’s location correctly.(Basic Rules, p. 76)

Depending on the circumstances the attack may auto-miss (you loosed the arrow in the wrong direction) or could be made with disadvantage per "unseen attacker" rules above.

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Special cases that likely need their own question.

Rulings on cases of tremorsense and blind sight will conform to those specific rules .

Answer 3

The rules on cover indicate that it will not block ranged attacks.

The rules on total cover (PHB p. 196) do not give examples for cover. But since the difference between half, three quarters, and full cover is only in the covering percentage, the other examples can be considered.

The examples are:

The obstacle might be a low wall, a large piece of furniture, a narrow tree trunk, or a creature, whether that creature is an enemy or a friend.

And:

The obstacle might be a portcullis, an arrow slit, or a thick tree trunk.

All of these block movement and most (but not all) block sight as well. There is however no example of anything that blocks sight but not movement.

The rules on vision and light (PHB p. 183) describe things that block vision.

A heavily obscured area - such as darkness, opaque fog, or dense foliage - blocks vision entirely.

A wall of sand, which is specified to block sight, but not movement is in the end much like dense foliage which is named under vision but not under cover, suggesting that it will provide disadvantage due to sight but not block projectiles.

Furthermore, other spells creating walls like this specify that they block projectiles, such as wall of force and wind wall.