We were fighting Vecna himself, and at one point had a *[prismatic wall](https://www.dndbeyond.com/spells/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](https://www.dndbeyond.com/magic-items/4738-ring-of-x-ray-vision)* states:

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> 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.
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So can someone see through a prismatic wall using truesight/x-ray vision?
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**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.
*Prismatic wall*:

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> A shimmering, multicolored plane of light forms a vertical opaque wall
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Truesight: maybe indirectly
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Monster Manual (p. 9):

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

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> Furthermore, the monster can see into the Ethereal Plane within the same range.
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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*](https://www.dndbeyond.com/spells/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](https://rpg.stackexchange.com/questions/195034/can-i-target-someone-who-i-can-see-in-a-mirror?rq=1) 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
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The [*ring of X-ray vision*](https://www.dndbeyond.com/magic-items/4738-ring-of-x-ray-vision) says:

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> 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.
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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](https://rpg.stackexchange.com/q/72780/23547).

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.