Can Truesight see through an area that is heavily obscured?

Question

Truesight states:

A monster with truesight can ... see in normal and magical darkness, see invisible creatures and objects, [and] automatically detect visual illusions ...

Heavily obscured is created by a variety of effects as noted in its description:

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

Now, we know that Truesight can see through darkness of any kind, and so if the heavily obscured area was created by darkness, Truesight would be unaffected.

However, there are a variety of game effects that create a non-darkness-related area that is heavily obscured such as the spells below:

Fog Cloud

The sphere spreads around corners, and its area is heavily obscured.

Cloudkill

Its area is heavily obscured.

For example, if a mezzoloth were to cast Cloudkill on itself to hide from a player using the Shield of the Hidden Lord's Truesight ability, could the Shield see it?

Can Truesight see through an area that is heavily obscured?

Answer 1

Truesight can see through exactly the things it says it can.

There’s an oft quoted phrase that applies to your question, “spells do what they say they do”. In this case, we’re talking about a special sense, which the rules define for us. Truesight allows you see:

• in normal and magical darkness
• invisible creatures and objects
• visual illusions
• the original form of a shapechanger or a creature that is transformed by magic.
• into the Ethereal Plane

If the thing obscuring your vision isn’t on this list, truesight doesn’t help.