As we advance through terrain month, possibly at only half speed, we of course must cover the topic of hazardous terrain. Because bullrushing someone into tall grass doesn’t have the same impact as pushing them off a cliff. By hazards, I don’t mean actual “hazards” as outlined in the core rulebook, but rather dangerous terrain features. This is one of the fastest go to options for completely devastating an opponent far stronger than the players. Hazardous terrain is the most exciting, but also most difficult to use. While a battle on a rope bridge over a 500 ft chasm is incredibly exciting, a 3rd level hero who falls is pretty much dead, barring a feather fall or similar effect. You wouldn’t include a trap that deals 20d6 damage, so you probably shouldn’t include a fall that does the same. But at the same time that doesn’t mean you have to avoid using those exciting locals, it just means you have to be careful how you use them.
A fight at the edge of a cliff could spell instant death, unless there happens to be an outcropping 20 or 30 ft down that can be landed on. The peril and threat o falling is still real, but now it means that instead of dying from a fall, the player has to spend a few turns climbing back up to get back in the fight. The rope bridge over a chasm? Add water at the bottom and it becomes a much less lethal drop that threatens to carry away its victims (likely to another convenient adventure spot). Battling atop uneven stones adrift in a tide of magma? Just make it ankle deep, reducing its damage considerably but keeping the threat very real. Thick vines and briars that entangle and damage anyone forced inside? Those are actually perfect on their own. The important thing is to keep the threats more balanced whenever possible. My recommendation is to treat dangerous terrain as a custom made trap, base the CR on it’s difficulty to avoid with a save (or no save), and on the average damage. This means that those deadly threats are now worth experience and can be quantified. This also allows us to scale up threats. Level 10 heroes don’t really have to fear 2d6 fire damage so shallow magma isn’t a huge threat, but deep magma is still intensely fatal. But by treating it like a trap we can perfect average damage for it to deal based on the level, and then explain that this particular depth and heat of lava deals this damage. And likewise if thorns and brambles have lost their kick, add some poison to them to spice things up and keep the effects relevant.
So that covers how to use hazardous terrain, but when should you use it? In my opinion, very frequently. One of the best ways to make a fight unique and exciting is the creative ways you can battle and best your opponent. I think that almost every combat encounter should include some terrain that does something. Whether it’s a lower level you can shove someone down to, dangerous or deadly spaces that harm those inside or more esoteric instances like stationary spell effects or squares that bulrush, grapple, trip, or disarm foes. The important thing is that there’s some reasoning and consistently. If random lighting strikes deal 2d10 damage at level 5 then by level 20 a random lighting strike should still be 2d10 unless it’s in some way magical or more potent lightning. Whenever a fight starts, think about what is around, how it can be used and what it can do, and make sure your players know those options are there.
Hopefully this helps you design encounters that are riveting, without being devastating and campaign ending. And remember, where a fight takes place is just as important as who is fighting.