Role-playing Games Stack Exchange is a question and answer site for gamemasters and players of tabletop, paper-and-pencil role-playing games. It only takes a minute to sign up.
Sign up to join this community
On your turn, you misty step 30 feet straight up. Can you make an attack before you start falling, or will you immediately begin to fall?
For example, a player wants to reach a flying enemy to knock him prone, then feather fall safely to the ground. Is he in reach long enough for all his attacks to go through? Will he fall before he can cast feather fall? (He used his bonus action on to cast a spell, so only cantrips during his turn.)
We can bypass all the discussion about when you really start falling, using the mechanics offered by a readied action:
When the trigger occurs, you can either take your reaction right after the trigger finishes or ignore the trigger. Remember that you can take only one reaction per round.
If you were to ready a melee attack, with a trigger for when you misty step, then you'll be able to use that melee attack right after you misty step.
To me this is an elegant solution by RAW that also contains some balance: only stuff that can be done by a Readied Action, cost your reaction... Your DM may also feel that disadvantage may be appropriate in this case.
As covered by this question, since you are using your readied action on your turn for an attack, you can use the Extra Attack feature by RAW.
Xanathar's Guide to Everything says so, on p.77:
When you fall from a great height you instantly descend up to 500 feet. If you're still falling on your next turn you descend up to 500 feet at the end of that turn. This process continues until the fall ends.
Emphasis mine. When you fall from anything more than a couple of feet, you become subject to the falling rules. You get enough time to react to the fact that you're falling with a reaction (for things like Feather Fall) or time to react to the fact that you're landing with a reaction (for reducing damage), but there is no in-between time.
You cannot do anything WHILE you're falling unless you fall for over 500 ft. In that case, you have a round to do something before you fall another 500 ft, and again each time until you land.
Attacks taken as a reaction to you either having made it up that far or beginning to fall are possible.
On your turn,
