True Seeing into Ethereal plane in Fog Cloud, what is seen?

Question

True Seeing spell allows, among other things, this:

and can see into the Ethereal Plane, all out to a range of 120 feet.

Assume that True Seeing doesn't allow one to directly see through the Fog Cloud fog, which is actual conjured fog and not an illusion, and creates a heavily obscured area. I believe this is a common ruling, not a contested topic, and anyway this whole question becomes moot otherwise.

But, Fog Cloud doesn't say it extends to the Ethereal Plane. So, considering above, what does the creature under True Seeing effect see in a fog cloud? Most importantly, does this give any advantage, be it for fighting or moving or anything, over other creatures in the Fog Cloud?

Or, is it simply so, that what ever may be seen on the Ethereal Plane does not reflect what is going on in the Material Plane? Or the fog in fact does heavily obscure the Ethereal Plane, too? Or what?

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I suppose related matter is, how are permanent obstacles like walls seen with True Seeing on the Ethereal Plane. My understanding is that they exist as "ethereal material" there, but I am not sure. Fog Cloud creates a temporary spell effect on the Material Plane though, so possibly different.

If this question can't be answered within 5e material, drawing from earlier edition rules or lore is ok.

Answer 1

The way I interpret true seeing is that if you can see an area, then you can see any ethereal creatures or objects in that area (among other things). If you're in a fog cloud, or wearing a blindfold, or have a bag over your head, you can't see at all, so your true seeing is functionally blocked as well.

As far as walls go, we're told that material objects seen from the ethereal side are "muted and indistinct", "shadowy", and that they don't "hamper movement", but nothing says you can see through walls simply by being on the ethereal plane. I see no reason truesight would be different.

The DMG doesn't go into a ton of detail about other planes, but one of my favorite 3rd Edition resources was the Manual of the Planes, which was very specific about this:

To viewers on the Ethereal Plane, objects on the Material Plane are foggy, indistinct, and almost translucent. Such objects block line of sight and provide concealment, but not cover. An ethereal observer can't see through a wall on the Material Plane unless he pokes his head through it first. An ethereal character whose eyes are totally within a Material Plane object cannot see.

That doesn't exactly address the question you're asking, but I think it's clear that the intent is that your field of vision doesn't change when you're seeing into the other side, only what you can see within that area.

Answer 2

Nothing more, unless there are ethereal beings in the cloud

According to the DMG, the Material plane could be seen from the Ethereal plane:

From the Border Ethereal, a traveler can see into whatever plane it overlaps

The Ethereal plane does not duplicate things from the Material plane. Instead, you can see one plane being on another plane. You can always see the Material plane from the Ethereal plane, but not vice versa, except with the aid of magic:

the Ethereal Plane is usually invisible to those on the overlapped planes, except with the aid of magic

The True Seeing spell gives you an ability to see the Ethereal plane, but this very ability doesn't reveal any new information about the Material plane.

Answer 3

If you can't see in the Material Plane, you can't see into the Ethereal Plane either

Truesight (such as that granted by True Seeing) gives you the ability to see into the Ethereal Plane, but your point of view is still located in the Material Plane. This means that any (non-illusory) opaque object or effect in the Material Plane blocks your vision as it normally would. As explained in the rules for vision and light, opaque fog blocks your vision entirely:

A heavily obscured area—such as darkness, opaque fog, or dense foliage—blocks vision entirely. A creature effectively suffers from the blinded condition when trying to see something in that area.

Truesight allows you to see such obstructions as they truly are: opaque. Hence, a creature with truesight in a Fog Cloud sees the same thing as everyone else: dense fog and nothing else.

Here's an analogy: imagine an ability that allows you to see through wood. This allows you to see what is on the other side of a wooden wall, but not if there is a stone wall in front of the wooden wall blocking your vision. The ability to see through wood doesn't override your inability to see through stone. Truesight allows you to see through the veil between the Material and Ethereal Planes, but it doesn't grant you the ability to see anything through opaque obstacles.