Are there any rules relevant to involuntary movement and effects other than Opportunity Attacks?

Question

PHB p. 195 states that involuntary movement does not provoke opportunity attacks:

You also don't provoke an opportunity attack when you teleport or when someone or something moves you without using your movement, action, or reaction.

Is there a general rule somewhere about involuntary movement and how it interacts with other effects?

The question arose at our table about what happens if a creature is dragged via the Thorn Whip spell across an area of Spike Growth. Does the involuntarily moving creature take the damage from the spikes as if he'd voluntarily moved through them?

Logically, yes, if you're being dragged through spikes, you take damage from them as if you'd walked through them. But logically, if I drag you past my ally, my ally would be able to make an Opportunity Attack, likely more easily than if you were moving under your own power. Presumably that is not allowed for balance reasons rather than versimilitude reasons, so I wonder if the same should apply to other effects?

Answer 1

The is no general rule (that I can find) regarding involuntary movement.

However, Jeremy Crawford - lead rules designer for 5e - has said:

When a spell's description uses "enter" in relation to an AoE, the entering has to be voluntary only if the text says so.

Answer 2

No, there is no general rule about involuntary motion, it entirely depends on the spell.

There is, however, a general distinction between when a spell talks about a creature moving into versus a creature entering the spell's area.

Movement is a defined game mechanic. When a creature moves, it's doing so of its own volition:

On your turn, you can move a distance up to your speed.

Spells like prismatic wall and guardian of faith specify effects when a creature moves to within a certain range.

Any creature hostile to you that moves to a space within 10 feet of the guardian for the first time on a turn [...]

By contrast, most area of effect spells don't use the word "move," instead they use the phrase "enters the spell's area," like cloudkill:

When a creature enters the spell's area for the first time on a turn or starts its turn there, [...]

While "moving" is a specifically defined game mechanic, "entering an area" isn't, so it only has the meaning the phrase naturally does in the English language, which doesn't require it to be voluntary. You can be pushed into a moonbeam and immediately suffer the consequences.

The Sage Advice Compendium specifically clarifies this point:

Entering such an area of effect needn’t be voluntary, unless a spell says otherwise. You can, therefore, hurl a creature into the area with a spell like thunderwave. We consider that clever play, not an imbalance, so hurl away!

In this specific case, the spell spike growth says:

When a creature moves into or within the area, it takes 2d4 piercing damage for every 5 feet it travels.

If a creature is pulled by thorn whip or otherwise moved by someone else, it isn't using its movement, and it doesn't take damage from the spikes.

Answer 3

The spell Spike Growth will damage a creature moving into or within its area of effect even if the movement is involuntary, such as the effect of forced movement from the spell Thorn Whip.

The Spike Growth spell description says:

When a creature moves into or within the area, it takes 2d4 piercing damage for every 5 feet it travels (PHB, pg 277)

There is no general rule regarding involuntary movement and there is no caveat in the spells description which might limit its effect in regard to involuntary movement.