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I am preparing an encounter of my players against a drow priestess of Lolth, a drow mage, and several regular drow and giant spiders. As the drow have been forewarned, they set up an ambush: The priestess cast Insect Plague on a spot 20 feet away from the entrance, and the mage cast Cloudkill on exactly the same spot. (I assume the insects aren't natural and don't get killed by the poison cloud). The regular drow and giant spiders stand behind the poison/insect cloud when viewed from the entrance.

Assuming the players arrive just after those spells have been cast, and before the Cloudkill starts moving, what would they see? The Cloudkill area is "heavily obscured", the Insect Plague area is "lightly obscured". Do they just see a green fog filled with insects on the first square after the entrance, while the drow and spiders behind the clouds are invisible? Or does the green fog hide even the insects? Once the Cloudkill area moved away, can the players see the drow and spiders through the lightly obscured Insect Plague?

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    \$\begingroup\$ @KorvinStarmast a very interesting setup indeed. But looking at Cloudkill description, there is no distinction between magical and non magical creatures: when a creature enters the spell's area or begins its turn here, it takes the damage. Hence, RAW I think that all the insects will die as they are inside the cloud. The DM has nonetheless the last word on it. \$\endgroup\$
    – Eddymage
    Commented Sep 5, 2020 at 14:31
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    \$\begingroup\$ The insects aren’t creatures and so can’t be harmed by Cloudkill or anything else. \$\endgroup\$
    – Dale M
    Commented Sep 5, 2020 at 23:09
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    \$\begingroup\$ Insect Plague fills a sphere with "swarming, biting locusts", and swarming locusts are highly audible. The players are going to hear them. (Before anyone makes a "spells only do what they say" argument to argue that the spell doesn't say it makes noise, please note that it also doesn't say it magically silences the insects. The whole "spells only do what they say" argument gets taken way too far to justify way too much.) \$\endgroup\$
    – user2357112
    Commented Sep 6, 2020 at 0:14
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    \$\begingroup\$ Totally agree with the insects being audible. On spells interacting with another in a "logical" way, I don't think that is implemented in 5E or any previous edition. A Fireball overlapping an Ice Storm or Sleet Storm spell neither has a diminished effect, nor diminishes the effect of the other spell. RAW and verisimilitude are two very different things. \$\endgroup\$
    – Tobold
    Commented Sep 6, 2020 at 5:54
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    \$\begingroup\$ @Eddymage from the spell - it doesn’t indicate in any way that they are creatures \$\endgroup\$
    – Dale M
    Commented Sep 6, 2020 at 10:28

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Heavily obscured areas block vision entirely.

Assuming that it is true that the insect won't die due to the Cloudkill spell, I think that the heavily obscured environment condition prevails on the lightly obscured, by description (PHB, pag 183) and meaning of words (heavy and light):

A given area might be lightly or heavily obscured. In a lightly obscured area, such as dim light, patchy fog, or moderate foliage, creatures have disadvantage on Wisdom (Perception) checks that rely on sight. A heavily obscured area—such as darkness, opaque fog, or dense foliage—blocks vision entirely. A creature in a heavily obscured area effectively suffers from the blinded condition (see appendix A).

The heavily obscured description says that the fog blocks the vision entirely, hence the players will not see the insects nor the drows and the spiders. After the fog starts moving, if I were the DM I would allow a WIS check (Perception) with an high DC to give players the opportunity to spot the insects and the drows, strongly depending on the direction of the fog: but this is RAI, it is up to you.

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