What is the guidance for setting the damage done by unusual difficult/dangerous terrain?

Question

With difficult terrain in 5e, how do you calculate the damage or special modifiers for terrain? I have found some related information, such as Pushing enemies into/over difficult terrain or cliffs in 5e?

I can also find information about the various damage sources for terrain (such as acid, spikes, lava, fire, etc). However what about other sources that are not written into the rules?

My question: How does the DM decide on damage/modifiers for terrain that is not standard? Are there any rules that specify this or guidance to help decide?

Here is a couple of scenarios I have recently come across that I was not sure how to handle.

• PC jumps through a window and attacks someone inside the building. I opted to have them take 1D4 slashing damage from the shards of glass as they went through, as well as an acrobatics check for jumping through the window.

• Creature is knocked back 10 feet from a spell, however they are standing beside a wall and get pushed into it. Creature rolls a dex save throw to land upright, otherwise they land prone.

I am most interested in written rules, guides, etc from the DMG/PHB/etc. I can only find generic ones related to typical sources of damage currently.

Answer 1

There are no specific rules regarding this, and the DMG leaves it up the to the DM. Your 1d4 for shattered glass seems reasonable (on par with a dagger); I might even bump that to 1d6 (have you seen what glass can do to someone? It's not a pretty sight). However, there are base guidelines for hazards and traps based on the character level and the amount of damage done. Those guidelines are reproduced from the 5e SRD (pg 201) below (pg 121 in the DMG). Note these values are also printed inside the 5e DM's Screen.

Damage Severity by Level

Character Level  Setback   Dangerous    Deadly
1st–4th             1d10        2d10      4d10
5th–10th            2d10        4d10     10d10
11th–16th           4d10       10d10     18d10
17th–20th          10d10       18d10     24d10

Since these values are for traps and might be a bit harsh if you're trying to determine the damage that, say, a bramble thicket might do, it might be enough to cut the values in half (round up).

Also note that even though this table shows 1d10 is a setback for 1st-4th level characters, that damage is likely to be closer to dangerous or deadly for level 1 characters who may not even have 10 hp to lose.

Answer 2

The examples you gave sound about right.

It's generally reasonable to consider something encountered abruptly, like a floor, window, or other object much like an improvised weapon, see PHB, p147.

As for acid, spikes, lava, fire, and so forth, often there are spells that provide some guidelines, although it depends greatly on the specifics. Did the PC get hit with a lavaball? That might be similar to burning hands. Acid? There's a cantrip for that! OTOH, falling into acid, spikes, lava, fire, etc., well, that's gotta sting!

In the DMG, the section on Wilderness Survival, beginning on p109, provides some guidelines for extreme cold, extreme heat, strong wind, heavy precipitation, altitude, desecrated ground, frigid water, razorvine, slippery ice, and thin ice. It's all fairly general stuff, though.

The Monster Manual gives some fairly good stuff. Of particular interest might be the lair actions of the dragons. For example, the red dragon's lair actions are:

• Magma erupts from a point on the ground the dragon can see within 120 feet of it, creating a 20-foot-high, 5-foot-radius geyser. Each creature in the geyser’s area must make a DC 15 Dexterity saving throw, taking 21 (6d6) fire damage on a failed save, or half as much damage on a successful one.
• A tremor shakes the lair in a 60-loot radius around the dragon. Each creature other than the dragon on the ground in that area must succeed on a DC 15 Dexterity saving throw or be knocked prone.
• Volcanic gases form a doud in a 20-foot-radius sphere centered on a point the dragon can see within 120 feet of it. The sphere spreads around corners, and its area is lightly obscured. It lasts until initiative count 20 on the next round. Each creature that starts its turn in the cloud must succeed on a DC 13 Constitution saving throw or be poisoned until the end of its turn. While poisoned in this way, a creature is incapacitated.

(MM, p99, under Red Dragon, Lair Actions.)

The other dragons have similar effects, but suited for that dragon's style. There might be other creatures that have associated terrain effects that might give some guidance.