I've been reading up on druid spells for future use and was quite interested in the crowd-control abilities of both Transmute Rock and Wall of Thorns, especially since Transmute Rock doesn't require concentration.

Both of the spells have a similar clause

Wall of Thorns (PHB, p. 287):

A creature can move through the wall, albeit slowly and painfully. For every 1 foot a creature moves through the wall, it must spend 4 feet of movement.

Transmute Rock (XGtE, p. 169):

The ground in the spell’s area becomes muddy enough that creatures can sink into it. Each foot that a creature moves through the mud costs 4 feet of movement,

So my question is, if they were cast together, would a single feet of movement costs 8 feet of movement(or possibly 16) instead of 4? One could argue that WoT mentions solid ground, but you could make the mud only 5-10 feet deep while the thorns are 10-20 feet high.

I also have a side-question to this, if they did work together, would you rule it to be difficult terrain or would there be the need for a third spell like Erupting Earth to turn the terrain difficult? Or would the mud already be considered the terrain and therefore it wouldn't be possible to actually turn the terrain any more difficult.

  • \$\begingroup\$ Also, I would ask the question on whether or not the mud counts as difficult terrain separately. We try to keep posts to one question at a time. \$\endgroup\$ – David Coffron Oct 6 '18 at 14:21
  • \$\begingroup\$ Sorry Rain. I just realized that thus is not a duplicate. I will revoke the closure. Hope you get answer soon! \$\endgroup\$ – David Coffron Oct 6 '18 at 14:41
  • \$\begingroup\$ Related: Wall of Sand on Difficult Terrain \$\endgroup\$ – David Coffron Oct 6 '18 at 15:05
  • \$\begingroup\$ Ah, sorry about that. I'll remember it for the next time I have a question. \$\endgroup\$ – Rain Oct 7 '18 at 7:26

Every foot costs 8 ft movement

You move through wall? Spend 4 ft. You move through mud? Spend 4 ft.

The effects are from separate spells. They don't turn the terrain into difficult terrain, because the spells don't say so, so they stack.

Another interpretation would be 7 ft per foot movement. This is true if you interpret the 4 ft movement cost as "extra 3 ft cost", but the wording for diffcult terrain uses "costs 1 extra", which both these spells do not, so it's a simple 4 ft substraction for each effect.

RAW, you spend 8 ft per foot movement through wall and mud.

Bonus: If you cast Transmute Stone first, then you can't cast Wall of Thorns because the surface is not solid anymore. Casting Wall of Thorns first allows you to cast Transmute Stone later, because there is no restriction other than requiring an area of stone.

  • \$\begingroup\$ Small concern: if you cast Wall of Thorns first and then Transmute Stone, the ground under the wall of thorns is no longer solid. Doesn't that mean the area is no long a valid target for Wall of Thorns? Jeremy Crawford has indicated that usually when that happens, the now invalid spell is suppressed. \$\endgroup\$ – Gandalfmeansme Oct 6 '18 at 21:33

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