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?