When a character with no fly speed ends their turn in the air, when should they start falling? [duplicate]

Question

We had a situation last session where I (sorcerer/monk) ended my turn mid air after striking a flying monster trying to flee. It was already 100 feet high so I used a quickened Thunder Step to teleport 90 feet up on the wall and step of the wind to jump from it and strike the monster with two melee attacks. It wasn't enough to finish it and I was left mid air.

I thought that I would immediately fall down and be okay since it's a forced movements and thus does not trigger an Opportunity Attack and thanks to the Slow Fall feat I wouldn't take too much damages. But The DM ruled it as the monster getting to hit me before I fall since his turn was next so I was knocked unconscious and died of the fall (no Slow Fall when you're unconscious).

I'm not mad at the DM for making this choice, I knew it was a reckless action when I took it, but it got me thinking when should a character start falling when they ends their turn in the air.

Answer 1

The Player's Handbook gives no guidance.

The basic rules for falling say:

A fall from a great height is one of the most common hazards facing an adventurer. At the end of a fall, a creature takes 1d6 bludgeoning damage for every 10 feet it fell, to a maximum of 20d6. The creature lands prone, unless it avoids taking damage from the fall.

Xanathar's Guide to Everything has the answer, and it's probably up to the DM.

In the optional rules for falling in Xanathar's Guide, we see:

The rule for falling assumes that a creature immediately drops the entire distance when it falls.¹

Notably, this rule is optional, or rather, an optional ruling on a previous rule, so it is between you and your DM to establish which rules you are using. Further, if you decide to use this rule, then it is still up to the DM to determine when "falling" really begins. Whenever "falling" begins, the optional rules here say that you immediately drop up to 500 feet.

I would have ruled that you fell the full distance before the end of your turn. I would make the argument that the falling began at the end of your attack action, and that you dropped the whole distance immediately. But your DM may rule differently, and you should discuss this with them.

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¹Note, when it says "the rule" it is referring to the rule for falling from the Player's Handbook cited in the first section of this answer. Xanathar tells us that the particular rule from the PHB assumes you fall the entire distance immediately, and goes on to give the optional rule for breaking it up into 500 foot blocks. But this ruling still appears in an optional rule section, even though it is not the particular rule being referred to as optional. It is still optional, since all of XGtE is optional. So ask your DM.

Answer 2

The rules give no guidance

This answer explains that well enough. The most important thing to note is that even when using the XGE rule, we only have guidance as to how fast one falls, not when one falls.

The following have worked well in play

Falling, obviously, comes up rather a lot in actual play. Having played quite a bit of 5e at this point, the following have worked well:

• A creature who finds themself unexpectedly without floor falls immediately after whatever check or saving throw lets them not fall or fall less far. So, like, for pit traps you make your Dex save and then fall if you fail.

• A creature who expects to fall (and is eligible, e.g. not on an invisible platform or something) may fall as, essentially, free movement, taking their actions or reaction wherever they'd like along the way. If an action requires especially precise timing or is made more difficult by falling, the DM may require some sort of check or confer disadvantage, but falling attacks are cool and fall damage is usually penalty enough.

• A creature who jumps or otherwise achieves significant upwards momentum does not fall until they reach the end of their jump or projected trajectory. A jump that exceeds a creature's movement for the round leaves them effectively in hang-time until the next round, at which point more of their movement is used to continue the jump.

• All eligible falling must happen by the end of the turn on which falling starts. Falling isn't usually left until the end of the turn, though unless unexpected it can be, but it must all happen before the next turn starts. The only exception is for rare, particularly long falls, which are broken up as per Xanathar's, and aren't unexpected after the first 500 feet.

Answer 3

From Xanathar's Guide to Everything:

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.

Keep in mind that Xanathar's itself mentions that it is an optional source to help DMs.

Instant is instant, as soon as you no longer have jump to propel you further in the air, you fall, that happened on your turn, not after your turn.

For a comparison. If you had used your item interaction while in the air to pull out a coin and let go of it, would it fall on your turn, or would it not fall until the next creature acted?