Do either truesight or x-ray vision see through a prismatic wall?

Question

We were fighting Vecna himself, and at one point had a prismatic wall up between us and Vecna. The prismatic wall says it creates "a vertical, opaque wall."

Vecna's powers include truesight, and presumably x-ray vision as well (since the artifact Eye of Vecna allows its bearer to use x-ray vision), and the DM thought Vecna would be able to see through the prismatic wall and target us. A ring of x-ray vision states:

While wearing this ring, you can use an action to speak its command word. When you do so, you can see into and through solid matter for 1 minute. This vision has a radius of 30 feet. To you, solid objects within that radius appear transparent and don't prevent light from passing through them. The vision can penetrate 1 foot of stone, 1 inch of common metal, or up to 3 feet of wood or dirt. Thicker substances block the vision, as does a thin sheet of lead.

So can someone see through a prismatic wall using truesight/x-ray vision?

Answer 1

Neither effect allows you to see through a Prismatic Wall

Prismatic Wall has this effect:

A shimmering, multicolored plane of light forms a vertical opaque wall

Truesight

Truesight allows you to

see in normal and magical darkness, see invisible creatures and objects, automatically detect visual illusions and succeed on saving throws against them, and perceive the original form of a shapechanger or a creature that is transformed by magic [...] see into the Ethereal Plane

It does not allow you to see through an opaque barrier. While you can see into the Ethereal Plane, what you are looking at is not on the Ethereal Plane, and if you could see them from there does not matter, as neither are you.

X-Ray Vision

X-Ray Vision (as in the ring) allows you to

see into and through solid matter (...) To you, solid objects within that radius appear transparent and don't prevent light from passing through them

The wall is not a solid object or solid matter, as it is made of insubstantial light that you can walk through, which you could not if it were solid. So X-Ray Vision does not allow you to see through it, either.

Vecna

Could you use either as a player character if you tried to look through a Prismatic Wall? No.

Could Vecna? Not by the official stat block that has been printed in the Vecna Dossier (where he does not even have X-Ray Vision, as he does not have the Hand or Eye of Vecna, he is said to be questing for them). However, the lore section there states that

More recently, Vecna decamped to the Outer Planes, where he grew so powerful that he became a god.

And there are no printed rules for gods. Even without invoking their right to overrule anything, the DM can rule that he could see through the wall. And even for pre-apotheosis Vecna, they of course can rule so, because it's Vecna and because of their right to overrule. Your DM in any case did rule so, so your Vecna could.

Answer 2

Truesight does not help - a Prismatic Wall is an Abjuration, not an Illusion, and neither involves the Ethereal plane nor shapeshifting.

X-ray vision is less clear - Prismatic Wall is defined as one inch thick, which definitely fits within the realm of X-ray vision. However, X-ray vision states "solid objects", which I'd argue a Prismatic Wall isn't, being defined as "a shimmering, multicolored plane of light ". As Prismatic Wall is a 9th level spell, so I'd tend to argue in favor of it doing more rather than less.

However, I'd say the rules are unclear enough that a DM ruling is required. And Rule 0 is always a thing - if the DM wants Vecna, one of the top BBEGs in D&D, to be able to see through something, then Vecna definitely can.

Answer 3

RAW, vague.

True sight and x-ray vision could either be argued to see through Prismatic Wall depending on various definitions of light, matter, ethereal, etc. However, D&D 5e is not a physics simulator. Neither explicitly states they see through 'planes of shimmering light' or w/e, yet they both talk about seeing through all kinds of stuff, so it's basically an argument either way.

Whenever there's an argument about anything in D&D, it comes down to the DM's decision. That's in large part why you have a DM (as well as to apply structure and motivation to the story). What a DM decides is reasonable in that situation is up to the DM and their table's expectations, but,

Should Vecna be able to see through his Prismatic Wall?

Yes.

He is Vecna.

Answer 4

Prismatic wall:

A shimmering, multicolored plane of light forms a vertical opaque wall

Truesight: maybe indirectly

Monster Manual (p. 9):

A monster with truesight can, out to a specific range, see in normal and magical darkness, see invisible creatures and objects, automatically detect visual illusions and succeed on saving throws against them, and perceive the original form of a shapechanger or a creature that is transformed by magic.

Nothing in this implies that truesight allows one to see through an opaque wall. However, truesight also grants:

Furthermore, the monster can see into the Ethereal Plane within the same range.

And here is where it gets complicated. Nothing in the description of prismatic wall says or implies that any layer of the wall is made of force, and thus the wall does not exist on the Ethereal plane. However, the characters are (presumably) living, and the DMG says that "[living beings] on the [Material] hamper the movement of a creature in the Border Ethereal." The Etherealness spell states that "You can see and hear the plane you originated from, but everything there looks gray, and you can't see anything more than 60 feet away." That is, someone on the Border Ethereal can see a version of the overlapping Material plane (although the objects appear as muted gray forms) and living creatures on the Material create impassible physical barriers on the Ethereal.

Thus it seems reasonable for a DM to rule that Vecna could use his truesight to see into the Border Ethereal, where projections of the characters appear in gray tones and where the wall does not block his sight. Since Border Ethereal locations 'map one to one' with the Material, it is then possible to rule that he can use this indirect sight to target characters on the material plane, since he can, in effect, 'see their location' by noting the location of their forms that are visible on the Ethereal. It is certainly not RAW, but it is also not an unreasonable interpretation. If anyone could do this, Vecna could.

X-ray vision - no

The ring of X-ray vision says:

When you do so, you can see into and through solid matter for 1 minute. This vision has a radius of 30 feet. To you, solid objects within that radius appear transparent and don't prevent light from passing through them.

The ring specifically says that it can be used to see through "solid matter [and] solid objects"; it does not impart any ability to see through opaque substances that are not solid. It could not be used, for example, to see through an inch thick of liquid mercury, or a dense gaseous fog or smoke.

While the wall is opaque, the "shimmering plane of light" is not solid in any natural English usage of the word. Creatures passing through it may trigger damage or other effects, but it does not physically block their passage. And it is certainly not an object; it is a magical effect. Thus the ring's ability to make "solid objects...appear transparent" simply does not apply.