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Lets assume the character in question is immune to fire/suffocation/crushing damage from being immersed/wading in lava. What is their RAW movement rate? Is it just "difficult terrain" for wading? Is it "swimming" for immersion? "Burrowing"?

I can homebrew an answer to this without difficulty, so I'm not looking for suggestions along those lines and I only want what the rules are. Answers should be backed up by rules citations.

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    \$\begingroup\$ Does this account for the surface tension of the lava? Might they be running on top of the lava? \$\endgroup\$ – Drunk Cynic Oct 3 '16 at 22:47
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    \$\begingroup\$ Do you want to know generally or is there a specific situation? \$\endgroup\$ – GoodSirTheSir Oct 4 '16 at 1:53
  • \$\begingroup\$ Just a reminder that comments are for helping improve the post, answer posts are for answering, and chat is for chatting. The comments on this have been super chatty/answer-y and removing them hasn't discouraged new ones, so here's a reminder as well. :) \$\endgroup\$ – SevenSidedDie Oct 4 '16 at 3:12
  • \$\begingroup\$ @nvoigt I think I can homebrew lava movement rules for my campaign just fine. But the picture does not answer the question in the RAW context. That is what is unclear to me: how to rule movement in lava using 5e RAW. \$\endgroup\$ – Mindwin Oct 4 '16 at 12:27
  • \$\begingroup\$ Can you clarify if you are asking about realistic lava or "fantasy lava"? You're mixing the two in the question, and that is causing problems for people trying to answer it. If you do want a mix of them, please detail the properties of your game's "realistic fantasy lava" so that answers can be useful. Thanks! \$\endgroup\$ – SevenSidedDie Nov 4 '16 at 18:07
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Players would not sink into the lava.

In real life, if you were to trip and fall on lava you would not sink into it, but would rather lie on the top of the lava as you were cooked alive. This is because lava is denser (\$\rho=3000 kg/m³\$) than the human body (\$\rho=1000 kg/m^3\$)by a significant margin, even if said human is wearing heavy plate armor. Now if the character was a rock golem or warforged (\$\rho=7500 kg/m³\$) this would be a different story, as the rock golem would be roughly equal density and could swim while the warforged still would sink.

So it is not physically possible to swim through lava. But it might be similar to slogging through mud, depending on your weight and the size of your feet, how much you are carrying on your back, etc. It also could be possible due to the viscosity of lava for the average adventurer to run across the top of lava (especially if they are a monk) like some type of lizards run across the top of water. The biggest concern if you aren't immune to fire and the temperature is having your flesh melt from the heat. And you might want to do something to prevent dying from toxic fumes, namely sulfur dioxide.

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    \$\begingroup\$ Garret, the question premises assume the character is immune to fire damage. \$\endgroup\$ – Mindwin Nov 4 '16 at 17:44
  • \$\begingroup\$ As Gerret said, he would just WALK over it. Lava is 3 times denser than humanoids, even in molten state. otherwise: youtube.com/watch?v=mW56dMH0T2s \$\endgroup\$ – Trish Nov 5 '16 at 18:54
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    \$\begingroup\$ You should certainly not test whether players will sink into lava. \$\endgroup\$ – Polisurgist Nov 5 '16 at 21:46
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GM's Choice

This is definitely a situation where the GM gets to decide. If the players happen to have the right magic items or spells or whatever else to survive contact with lava for extended periods of time then it is up to the GM to decide what else happens inside that lava.

Impaired movement, difficult terrain, forcing the players to swim around in the lava... all of these are possibilities and it is up to the GM to decide which ones apply, as the GM is the individual with the most knowledge about how the lava/magma functions in this universe (even if that knowledge is entirely fabricated on the spot).

To the best of my knowledge, there is no specific rule on the books for walking through lava. 5th edition leaves a lot of rule-resolving up to the GM.

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  • \$\begingroup\$ humanoids can swim in water that is about as dense as themselves as they can submerge in it, but not in objects that are 3 times more dense than themselves. So if you should be able to survive both the toxic fumes AND the heat, you could just walk over it like wading through thigh high mud. \$\endgroup\$ – Trish Nov 5 '16 at 19:17
  • \$\begingroup\$ @Trish If you can find in the 5e sourcebook where they mention the density of lava/magma then I would be happy to read about it, and encourage you to post your own answer pointing that information out. Otherwise, my answer is valid. D&D is an approximation of our world, but that does not mean it follows the same laws of physics or that all matter there has the same properties as matter on Earth. \$\endgroup\$ – GreedyRadish Nov 6 '16 at 2:40

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