This chapter provides a set of additional rules that don't fit neatly into other sections of the guides. They aren't as necessary for basic play, but using them can enhance your experience with new possibilities and additional options. They are distinct from Optional Rules in that, while comparatively minor, these are still assumed to be in play during the course of a normal game.
With its detailed exploration system, most environmental effects are taken care of using Challenges like Environs or Traps. However, those are typically used for dealing with very specific hazards as the PCs adventure about. Many environmental effects are more constant and don't necessarily represent a threat; in fact, they may even provide a benefit. These also function outside of exploration scenarios; many are present within combat, for example, and may linger from scenario to scenario.
They work very similarly to conditions, except that you, the DM apply them as needed instead of relying on entry text. Each effect will provide suggestions for when to apply each particular thing, but the final judgment is in your hands.
Being drenched in water is frequently unpleasant and comes with a number of risks, though it also conveys certain benefits. Clever characters might look for ways to drench themselves or even a foe in order to gain some advantage from these effects, but mostly it's something unavoidable whenever you're forced to take a swim or muck about in the rain.
Wet entities suffer +1 Weakness to Lightning and Cold. Such entities, or entities climbing wet surfaces, suffer -1 dice level to their opposed roll to successfully climb (usually Acrobatics or Inflict) and -1 dice level to most other Acrobatics checks, including anything related to balancing or maintaining a Grab. Continuous application of water (such as from rain) prevents the use of mundane fire-based light sources, such as Wax Candles, Torches, Oil Lanterns, and Belt Lamps.
However, wet entities enjoy benefits from their slippery state, gaining +1 dice level to Acrobatics rolls to escape from Grabs. The water also provides temporary protection from high temperatures; the entity gains +1 Resistance to Heat and Fire. However, suffering any Heat or Fire damage then dries the entity enough to make them no longer wet, at least until new exposure.
Use these rules during a rainfall or a swim, or after such a drenching until the entity has had time to properly dry off. Consider environmental effects when determining if an entity is likely to have dried; a PC that changed clothes and then walked around in a desert will dry much faster than one wearing soaked layers in a humid marsh.
Do not apply these effects to entities that are inherently assumed to have this trait. For instance, many Aquatic enemies and/or those mostly made of water already have Lightning Weakness in their entries, and walls slick with slippery substances have a higher level and/or Endure or Acrobatics dice to reflect this. Only apply these effects to entities that aren't normally wet.
You'll want an entity to be pretty solidly wet before applying these effects. A bit of sweat or dew, or a small spray of water (such as from a tossed waterskin) is unlikely to be sufficient.
You may wish to stop applying these rules at higher levels (such as once players receive their Destiny). At this point, PCs are dealing with effects so powerful that mundane elements like this no longer apply. You probably don't want a PC recently dunked in water to get much Resistance from a dragon's breath attack, for instance, and a little water isn't going to really impede a high-level character's ability to scale a wall. Go with what feels right for the overall power level of your adventure.
Adventuring is a dirty business. At some point, nearly every adventurer has crawled through a disgusting sewer, muddy swamp, or viscera-filled tomb of undead, and that's before considering the weeks spent on the road. The lifestyle can result in quite the accumulation of dirt and stench, none of which does much for the reputation of adventurers within civilization or one's overall health.
These rules should apply mostly to PCs that have been adventuring in particularly galling locales, or who have been traveling on foot for at least three days without opportunity to clean (such as in streams, ponds, or with the use of various magics), though the time range may vary greatly depending on the environment and method of travel. Generally, PCs on Mounts or Vehicles should avoid becoming Filthy except in pretty extreme circumstances. You should also use these rules after a PC is hit by a particularly gross attack, especially one involving viscous fluids or terrible stenches.
Once PCs get access to Rituals, stop applying these rules from basic travel; at this point, PCs have enough general magic access to trivially clean from mundane circumstances, and any charm from needing to bathe after walking between cities has likely faded. Save it for the immediate aftermath of especially disgusting events or locations, mostly for the added flavor.
You might include additional effects in stealth scenarios. Being covered in mud might work like camouflage in some situations, granting a +1 dice level bonus to Endure against Seekers. On the other hand, scent-based Seekers might have an easier time discovering a smelly PC, resulting in -1 dice level in these circumstances. Don't overuse this, but have fun with it.