by Mike Mearls

Back in the first column of this series, I talked about asymmetry, or the idea that terrain could hinder one side but not the other. Preferably, the player characters feel the pain (or annoyance) of an area but the monsters survive it unscathed.

This article looks at terrain that inflicts damage on anyone who crosses it, such as a rivulet of lava or a jagged, spiked rock. In particular, it shows how you can determine an appropriate Challenge Rating for such a threat. After all, if the terrain hampers the PCs, any encounters they must deal with there are more dangerous than normal. With increased danger should come a higher Encounter Level and the superior treasure and experience points that tag along with it.

Terrain Damage Types

The easiest way to determine what sort of damage a terrain feature should deal inflict is to look at the damage that traps inflict (see Chapter Three: Adventures in the DMG). Depending on the type of terrain or the environmental conditions, you might either have terrain that either uses an attack, requires a saving throw, or inflicts damage automatically.

Attacking Terrain

This sort of terrain effect must physically strike a player character to inflict damage. For example, a minor earthquake could cause the roof of a cavern to crack, fold, and drop debris down upon the party. If you see the PCs using their armor to survive a terrain feature when you play the scene out in your mind, it probably uses an attack bonus.

Designing this sort of terrain is easy. Pick a trap that uses an attack bonus and has the Challenge Rating you're shooting for. The PCs receive XP and treasure for defeating that Challenge Rating as normal. This assumes that they face the effect only once during an encounter.

If they face it about every other round, increase the CR by 2. If they face it every combat round, increase the CR by 4. This rough rule of thumb should keep the danger level balanced with the potential rewards.

Examples: Falling rocks, tumbling trees, boiling mud that pops and roils to spray an area with searing water, carnivorous plants that snap at animals.

Saving Throw

This type of effect is difficult to fend off with a shield or a stout suit of armor, but a tough, quick, or iron-willed player character might resist it or reduce its effects. If, in your mind's eye, you see the PCs dodge out of the way, use their mental focus, or draw on their toughness to survive an effect, it probably requires a saving throw.

As with attacking terrain, terrain that uses a saving throw effect should draw its Challenge Rating from a related trap listing. If the PCs face the effect every other round, increase the CR by 2. If they face it every round, increase it by 4.

Examples: Poison gas, extreme heat, lightning bolts from a storm.

Auto Hit

This sort of terrain inflicts its effects regardless of the player characters' saving throws or Armor Class. Only by avoiding the terrain or bypassing the effect can they dodge it. If you see an obstacle that the PCs must leap over or move around in your mind's eye, it probably automatically inflicts its effects on characters who come into contact with it.

This terrain type is a lot harder to judge than the others. As a rule of thumb, increase the Challenge Rating of a trap requiring an attack roll by 2 to determine the auto hit terrain feature's Challenge Rating. As described above, increase this by a further 2 points if the effect plays a role in half the combat (or every other round), and by 4 if it's a factor in the entire combat.

Examples: A collapsing ceiling, a river of lava, a flash flood.

Challenge Ratings and the Role of Damaging Terrain

Calculating a terrain feature's Challenge Rating requires more finesse than simply applying a number to it. In the aftermath of an encounter, you should judge if the terrain played a real role in the proceedings. For example, a fiery pool of lava could be a dangerous threat, but if the player characters lure the fire giant cleric out of it before attacking him, it doesn't play much of a role in the adventure. In this case, it makes sense to scale back the award for overcoming this obstacle, since it's confined to only part of the battlefield. The Challenge Rating guidelines given above assume that a terrain feature comes into play throughout an encounter area. Use the following modifiers if that isn't the case.

Terrain Covers . . .	Modifier
Three-quarters of encounter area	CR - 1
Half of encounter area	CR - 2
One-quarter of encounter area or less	CR - 4

Other Effects

If the traps don't have the specific effect you're looking for, you can always pick out a spell to use as the basis for the terrain's effects. The base Challenge Rating for a terrain effect that uses a spell is the minimum caster level for the spell - 2. Determine the spell's damage, save DC, and other effects by using the minimum caster level. If you want to use the spell effect with an attack bonus, double its Challenge Rating to determine its attack bonus.

An auto hit effect terrain feature is difficult to adjudicate in this manner. As a rule of thumb, increase the base CR by 2. However, in some cases, this increase doesn't reflect the danger of the effect. For example, a flesh to stone effect that doesn't allow a save probably kills a lot of the party members automatically. Obviously, the Challenge Rating should be much higher than normal, or the effect becomes unbalanced.

Describing Damaging Terrain

Determining the Challenge Rating, damage, attack bonus, or save DC of damaging terrain is the easy part. Once you've done that, you need to look at how to present the terrain in your game. A good, vivid description can do a lot to make damaging terrain more interesting. If you simply tell the players that their characters take a few points of damage each round, you might rob your campaign of a fun scene or memorable encounter.

Stunts and Damaging Terrain

Damaging terrain is a good way to push the party members to use stunts (see Chapter Four in The Book of Iron Might). You might create an encounter in which the PCs must walk across a narrow wooden beam to avoid a pit of boiling water. A red-hot, iron wall might be scalable only with a good Jump check. When creating terrain, look at the skill list in the Player's Handbook and think about how each of the Strength- or Dexterity-based skills could play a role in avoiding it.

Did you catch Part One and Part Two of this article?