What happens when a flying creature is knocked prone?

Question

As per the title. The specific example I'm thinking of is an Archer Battle Master Fighter using Trip Attack on a Druid shapeshifted into an Air Elemental floating 120 feet off the ground. If the answer is different for this and the more general case, then answer each separately.

As noted in the tags, I'm looking at strictly RAW here, not RAI or house-rules, so references to specific pages of the PHB are appreciated. If there are any official answers on this, links to those would be helpful as well.

Answer 1

From the Player's Basic Rules (v0.2), page 71:

If a flying creature is knocked prone, has its speed reduced to 0, or is otherwise deprived of the ability to move, the creature falls, unless it has the ability to hover or it is being held aloft by magic, such as the fly spell.

So a flying creature that is knocked prone will fall, unless it has the ability to hover, in which case it will remain where it is. In the specific case of the elemental, it is immune to the prone condition, so nothing will happen to it. But even if it wasn't, it has the ability to hover, so it wouldn't fall anyway. (Or, as SevenSidedDie puts it, double-nothing happens.)

Note that whether it can hover or not, unless the creature is immune to the prone condition the other effects of the prone condition apply. So the creature's attacks have disadvantage, and attacks against the creature have advantage if they are made from within 5 feet, or disadvantage if they are made from further away.