Falling onto a Creature vs Moving Around Other Creatures Rules

Question

So I was looking into the 'moving around other creatures' section in the PHB and found this

You can move through a nonhostile creature's space. In contrast, you can move through a hostile creature's space only if the creature is at least two sizes larger or smaller than you. Remember that another creature's space is difficult terrain for you. Whether a creature is a friend or an enemy, you can't willingly end your move in its space.

Then I remembered Tasha's Cauldron of Everything (p. 170) has a interesting section on what happens when you would fall into a creature's space:

If a creature falls into the space of a second creature and neither of them is Tiny, the second creature must succeed on a DC 15 Dexterity saving throw or be impacted by the falling creature, and any damage resulting from the fall is divided evenly between them. The impacted creature is also knocked prone, unless it is two or more sizes larger than the falling creature.

At first it looks like this just happens if you fall and would land on someone, but because of the phrasing of being able to "move through a hostile creature's space only if the creature is at least two sizes larger or smaller than you", you would just stop 5 ft above the creature and just... take fall damage because the fall ended, with no effect on the one you fell on?

Doesn't really make sense to me if that's the case but falling onto a creatures are weird enough already with Wavejumping being a thing. Is there anything I'm missing here or can you only fall into the space of a creature that is 2 or more size categories smaller than you?

Answer 1

Forced movement can force creatures together

The restriction against moving through a hostile creature's space only applies to willing movement. That is, it applies to movement in the sense of "on your turn, you can move a distance up to your speed" (along with other willing movement mechanics such as readied moves). There are many ways other than movement to change location, willingly or otherwise, and some of these can force a creature into another creature's space. In short, this rule is irrelevant when considering falling.

There aren't actually any rules for falling in the PHB

That might sound like a surprise, but here is the entire section on falling in the PHB/Basic Rules:

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.

This section describes what happens when you land at the end of a fall, but it doesn't specify when, how, or where you fall. That is left entirely up to the DM. XGtE adds some additional rules for falling, but doesn't really take any control away from the DM. For example, the rule about rate of falling doesn't specify that you fall straight down, so the DM could decide that your fall follows a parabolic arc if that makes sense for the situation, just as easily as they could using only the PHB.

So what about falling into a creature's space?

This leaves us with a bit of a conundrum when looking at the rule you quoted from TCoE for falling onto a creature. This rule says what happens "if a creature falls into the space of a second creature," but there's no falling rule in any of these source books that specifies how or if this can happen. However, based on the fact that the subsection is called "Falling onto a Creature", we can infer that the likely intent is that these rules should apply to any creature falling onto another creature. In other words, the TCoE rule appears to assume that when a creature falls onto another creature, the falling creature lands in the second creature's space and triggers the rule.

Ultimately though, the rules for falling are left mostly in the hands of the DM, who should make a ruling appropriate to the situation.

Answer 2

You can fall into a hostile creatures space, and take fall damage

When the rules talk about you moving here, movement is a defined term where you are spending your speed, that is something that you do actively. The preceding page 190 in the PHB, that opens the section you are citing explains this:

On your turn, you can move a distance up to your speed. You can use as much or as little of your speed as you like on your turn, following the rules here. Your movement can include jumping, climbing, and swimming. These different modes of movement can be combined with walking, or they can constitute your entire move. However you’re moving, you deduct the distance of each part of your move from your speed until it is used up or until you are done moving.

You are not moving in that sense when you are falling, as falling does not expend any of your speed, so the rules about not being able to move into a space controlled by a hostile creature do not apply to falling.

If you use the core rules, you will take normal fall damage from the fall. If you use the optional rules from Xanathar's, you use the procedure of checking if you happen to fall onto the creature in the space.