Terrain Rules

If a battle takes place in a large open field, a wide market square or a big cavern, each combatant can engage and disengage freely, and no open lava pit is waiting to swallow those who make a careless misstep. For the average battle, that's fine, but when you plan that big, climactic encounter, you'll want to add a few twists to keep the players on their toes. The following are simple rules to achieve that.

Basics

Special terrain adds one or more additional rules that apply to all creatures on the battlefield.

If you plan a large set-piece battle, you can deviate from that and split the battlefield into zones. All combatants in one zone are nearby to each other, and they are subject to the same special rule(s). It takes a move action to move from zone to zone, unless there is a special obstacle that requires a skill check (set a DC) or a special movement mode (like flight) to overcome.

For example, you could have a frozen lake and a ledge above it. To get from the lake to the ledge, a DC 20 climb check is required. To get from the ledge to the lake, a DC 15 jump is enough. You declare the lake as thin ice and slippery, while the ledge is higher ground (see below).

Some terrains have nastier specials. These work like monster nastier specials. Add them when you think the PCs deserve it.

Of course, you can combine terrains. For example, there could be pillars in the room providing cover, but they also act as conductors boosting lightning damage.

Simple Terrain

These only add a single bonus or penalty. (I should convert these to a table)

Charged

There will be a thunderstorm soon.

All combatants are vulnerable to lightning.

Crowded

Innocent bystanders everywhere.

It's impossible to use attacks that target multiple enemies (without harming bystanders and suffering severe consequences).

Curative

A lingering holy enchantment, or just fresh air?

Add one die (per tier) to all recovery rolls.

Cursed

An ancient curse makes it hard to shake off conditions.

Saves take a -2 penalty.

Crystal Cave

The cave is filled with shards that fall from the ceiling on strong vibrations.

When a critical hit is scored, all combatants take X damage.

Desecrated

Necromancy is strong here.

All combatants are vulnerable to negative energy.

Eerie

Something spooky is in the air.

Combatants cannot benefit from the escalation die, unless they are immune to fear (such as Paladins with the Fearless talent).

Foggy, Dark

Limited visual range.

-2 penalty to attacks against targets you are not engaged with.

Hard to Breathe

Recoveries only heal half damage.

Low Ceiling

Two-handed weapons (including bows) take a -2 penalty to attack.

Low Magic

All magic items count as one tier lower (adventurer-level items don't work at all).

Nastier special: Reduce the damage dice of magic spells by one tier.

Magma Chamber

The heat amplifies the power of all fire magic.

All combatants without fire resistance are vulnerable to fire.

Maze

The terrain allows combatants that are not engaged to flee from combat and disappear in the maze as a move action. Following them requires a difficult skill check.

Pillars

Plenty of cover to go around.

Combatants that are not engaged gain a +2 bonus to AC and PD.

Poisonous Fumes, Lightning Discharges, Insect Swarms, Burning Swamp Gas

Combatants must take a move action each turn to protect themselves (like breathe through a cloth or take cover) or take X damage. Creatures with resistance to the damage type are immune.

Slippery, Icy

All combatants have disadvantage on disengage checks (roll twice and take the lower result).

Spiked Pit, Hailstorm, Tornado

Outside of a few safe zones, you're likely to be hit by flying objects or step into something nasty.

If you take a move action, make a Dexterity-based skill check and take X damage if you fail. 

Thick Underbrush

Are you hacking at vines or at the enemy?

Attacks don't deal miss damage or miss effects.

Underworld

Death is close here.

Combatants can only fail ONE death save. After the second, you are dead.

Complicated Terrain

These add more complex rules and affect things like movement, engaging and intercepting.

Higher Ground

Higher ground is a smaller zone on the battlefield, never the entire field. Combatants on higher ground gain a +2 bonus to ranged attacks against combatants in other zones. Targets on lower ground count as "nearby" for the purpose of ranged attack reach.

Narrow Corridor, Narrow Bridge, Chokepoint

This terrain changes the engagement rules. Each side of the battle is split into two groups: front line and back row.

Only one or two normal-sized creatures can be in the front line, the others are in the back row. Combatants in the front line are engaged, combatants in the back row are not.

If two allies are in the front row, combatants in the back cannot engage. They must use a ranged or close-quarters attack or wait for an ally in the front row to disengage.

When a combatant in the front row disengages with a successful check, and there is no ally left in the front, a volunteer (or a random ally if there are no volunteers) is now the front row.

Combatants cannot move past the enemy front row with normal movement, they are always intercepted. This does not apply to teleports, walking on the ceiling or flight (if the ceiling height allows).

A single large creature fills the entire front row. Huge creatures can't enter the corridor (unless they can burrow).

Nastier special 1: Very narrow. Only one creature can be in the front line. Large creatures can't enter the corridor.

Nastier special 2: Low ceiling. Two-handed weapons take a -2 attack penalty.

Nastier special 3: Watch your back. If a combatant in the back row rolls a natural 1, the attack hits their own front row.

Thin Ice, Swamp, Quicksand, Lava

Watch your step. On a natural 1 on attack or disengage check, you are swallowed by the floor. You pop free from your opponents and take X ongoing damage until you climb out with a successful skill check. A save against the ongoing damage does not stop it, but it is reduced to half.

Nastier special: Ongoing damage / climbing out is a hard save / skill check.