13th Age doesn’t much care for terrain or environmental effects. There’s the Tracker and Swashbucker talents, and a bit on using traps in the core book. The GM Resource book has a solid chapter on using terrain and overcoming the stand-and-hit-each-other problem. Good advice, but no mechanics. So, here are some mechanics. This post is a toolbox more than anything, providing you with a starting point to create the terrain effects that match your situation.
The idea is directly inspired by D&D 5e’s lair actions. But whereas the lair actions are tied to boss monsters controlling their environment, terrain effects are tied to Icons, can appear anywhere, and can be taken over (or even created!) by PCs.
Adding Terrain Effects to Combat
The simplest way to add a terrain effect is to have it be there from the start. A terrain effect is roughly equivalent to a single-strength creature of its level. If the opposition owns it (see below), simply include that in your battle building calculations. If it is neutral, but starts out under the opposition’s control, it’s still worth something. Neutral active terrain effects make life harder for everyone and probably don’t affect the calculations.
There is, however, another option. Players can use one of their 6’s or 5’s on the icon relationship dice to create or conveniently discover a terrain effect. 13th Age is a high magic game, with Icons affecting the world itself with their will. That said, it’s entirely up to your GM (and the group) how often this would happen. The effect, as well as the locaiton, should fit thematically – utilising your relationship with the Lich King at a graveyard to cause zombie arms to pop up from below the ground and grasp at your enemies is cool. Calling on the same zombies to grab your enemies in a royal palace is a bit odd.
A 6 on a relationship die gives you ownership of the terrain effect. It can also give you ownership over a neutral terrain effect already present on the battlefield. A 5 provides a neutral terrain effect.
What does owning a terrain effect mean? Whoever owns the terrain effect chooses who counts as its enemies, simple as that. Neutral terrain hates everyone equally.
Mechanics of Terrain Effects
Terrain effects use the numbers from the Skill Check DCs, Trap/Obstacle Attacks & Impromptu Damage by Environment table on the page 186 of the core book. Depending on how far along in a tier your party is, use the Normal or Hard line. Handwavy, I know, but the convenience of using a single table from the core book beats individual write-ups.
Whenever monsters are forced to do skill checks, they roll a save instead: easy save replaces a normal check, normal save replaces a hard check, and a hard save replaces the ridiculously hard check.
Each terrain effect listed below is abstract, but comes with a number of examples. Modify the effect based on its flavor – change the damage type, the condition it inflicts, the defense it targets, or the skill it requires.
This is another things 13th Age doesn’t normally have. There’s two approaches here: if the terrain has a clear boundary, like ice-covered river, that’s the zone. Otherwise, if it originates from a single point, whether that point actually exists (unholy idol) or doesn’t (cloud of gas), the zone of effect is everything nearby this point.
Whenever a creature is on the edge of a terrain effect zone, a PC engaged with it can use a quick action to make whatever skill check they can get away with, to conclusively move their target into or out of the zone. Likewise, if a PC is on the edge of a terrain effect zone, a creature engaged with them can use a quick action to cause the PC to make a skill check or end up in or out of the zone.
Active terrain is almost a creature in its own right. It acts on its own initiative – give it an initiative modifier as you see fit, much like you would with a custom monster. No need to overthink this, +0 is generally fine.
Attacks PD of 1d3 random enemies within it, dealing damage.
Examples: lava eruptions (The Three, High Druid, Diabolist, Orc Lord), steam vents (Crusader, Dwarf King, Archmage, Emperor), statue of an angry god (Crusader, Diabolist, Priestess, Lich King), snake pit (High Druid, Prince of Shadows, The Three).
Moves in a predetermined fashion, attacking PD of everyone it encounters along its path, dealing damage.
Examples: rolling boulder (Orc Lord, Dwarf King), boulder perpetually rolling down a Penrose staircase (Archmage), swinging axe (Dwarf King, Crusader).
Attacks PD of 1d3 random enemies within it, those hit become stuck, save ends. If the target is already stuck, the terrain deals damage instead.
Examples: strangling vines (High Druid, Elf Queen), zombie hands (Lich King), sinkhole (Dwarf King).
Passive terrain affects enemies entering, ending their turn, or trying to do something within it.
Whenever an enemy ends their turn within this terrain, it attacks their PD. Those hit take damage, or become weakened or confused until the end of their next turn, depending on the polutant.
Examples: aerial poison (The Three, Prince of Shadows, Lich King), room on fire (The Three, Diabolist, Orc Lord, Great Gold Wyrm), spore cloud (High Druid, Elf Queen).
Whenever an enemy tries to move within this terrain, it must make a Dex skill check or lose the action.
Examples: patch of ice (High Druid), waist-deep swamp (The Three), tar (Orc Lord).
Enemies are vulnerable while within this terrain.
Examples: blood-soaked fields (Orc Lord, Crusader), sacrificial altar (Lich King, Diabolist).
Whenever a non-enemy starts their turn within this terrain, it can make a save against a condition affecting it.
Examples: sanctified ground (Priestess, Great Gold Wyrm), inspiring statue (Emperor, Elf Queen, Dwarf King).
This is obviously not an exhaustive list, and I’d love to see your suggestions on expanding it.