Can a creature with immunity to the prone condition voluntarily go prone, such as a ghost that wants to impose disadvantage on ranged attacks against it?
If so, does such a creature who went prone in this way still suffer from the negative effects of being prone: moving at half speed, spending half their speed to get up, and having disadvantage on attack rolls?
Had a similar question come up in a campaign I was running recently. I had to house rule it a no. Immunity isn't voluntary so a ghost cannot go prone just as a Paladin PC with divine health can't voluntarily become infected with lycanthropy. I look at it like the immortal man who wants to die but can't. The mind may try but the physical form can't actually make it happen.
The prone condition requires a creature to have an "top side" (head) and a "bottom side" (feet). This "bottom side" must physically interact with the ground to move the creature effectively, while moving any other way (like crawling) is less effective.
Combatants often find themselves lying on the ground, either because they are knocked down or because they throw themselves down.
If you cannot knock a creature down (because you cannot physically interact with it, like a ghost, or the creature has no top or bottom side, like a slime) it's unable to become prone.
The item description of ball bearings include:
As an action, you can spill these tiny metal balls from their pouch to cover a level area 10 feet square. A creature moving across the covered area must succeed on a DC 10 Dexterity saving throw or fall prone.
And the description of the spell sleet storm includes:
The ground in the area is covered with slick ice, making it difficult terrain. When a creature enters the spell’s area for the first time on a turn or starts its turn there, it must make a Dexterity saving throw. On a failed save, it falls prone.
That means the creature must physically interact with the ground to be able to go prone. If a creature never interacts with the ground, because it's naturally levitating or magically hovering, it cannot go prone.
