It was an extremely kind rejection to receive 🙂
I’m kind of okay with it, frankly: it means I can 1) post much of it it here on my blog without agonizing over editing it and making it correct (suckers! You’re getting the raw stuff!) and 2) I’m pretty busy with work these days, so anything that means less responsibility is ok by me.
Without further ado: how the sausage are made.
Fey terrain is similar to mundane terrain, but often more so: thorns are sharper, toxins more psychedelic, roots more grasping. The specific effects encountered depend on where you’re encountering the fey, of course.
Arctic terrain effects
This lightly obscuring fog looks like a simple mist, but consists of razor-sharp crystals of ice suspended in midair. A DC 10 Wisdom(Perception) or Intelligence(Investigation) check while investigating it allows a character to discover the hazard. Icemist banks tend towards the small size in areas of extreme cold.
Every five feet of movement icemist deals 1 cold damage and, if the victim takes the cold damage, 4 piercing damage. Any amount of fire damage destroys ice mist in its area, as do strong winds. Destroyed patches reform after 1 minute.
Coastal terrain effects
A cross between stable ground and a pitching ocean, these unquiet dunes heave with the tides and are extremely obvious to travelers. Creatures starting their turn or entering within a region of tidal sands take 10 bludgeoning damage and fall prone (a DC 10 strength saving throw halves the damage and negates the prone). A proned creature is pulled 10 feet towards the center of the patch, cannot move towards the edges for one round, and has disadvantage on this save for one round.
Desert terrain effects
A creature seen against a background of seeming sand — surrounded on all sides by at least 10’ of the sand — is extremely difficult to see. Attacks against them are made with disadvantage. This effect is negated if the attacker doesn’t use sight or can pierce illusions.
Any creature drinking water directly from this oasis gains the effects of a sanctuary spell (DC 10) for 24 hours and must make a DC 10 wisdom saving throw to leave the area. The waters are surrounded by the bones of carnivorous predators and the hum of herbivorous insects.
Forest terrain effects
Creatures must spend 4 feet of movement to move 1 foot through this thick and tangled space.
Blood spilled in the same space as these roots awakens them; note that the roots can travel quite some distance from their central tree, hundreds of feet at least. Creatures that end their turns or enter onto thirsting roots while at half their hit points or fewer risk being grabbed (DC 13 Strength or Dexterity saving throw), or if grabbed restrained, or if restrained taking 10 (3d6) necrotic damage. A given 5 foot patch of roots has AC 12 and 10 hit points, vulnerability to fire, immunity to bludgeoning, poison, and psychic damage.
These huge trees (statistics as animated trees but they do not attack) move when unobserved, particularly to create, change or obscure paths through their wood. Attempts to navigate an area with active crowding hedge-oaks are at +5 DC.
Grassland terrain effects
The whistle of wind through these reeds sounds like the pipes of a bard. Satyr’s panpipes are often made of songreeds. Anyone hearing a songreed increases their maximum hit points by 5 and their current hit points to match, until the end of their next short rest.
Mountain terrain effects
Stone permanently under a phasing effect, this stone can be phased through by creatures and objects as though not present. This effect can occasionally be one way, creatures taking 1d10 force damage per 5 feet passed “backwards”. The stone can be recognized with a DC 10 Intelligence (History) check.
Swamp terrain effects
This spongy sphagnum is difficult terrain. Each 5 feet of movement across its surface has a 25% chance of releasing a 10 foot radius cloud of bilious yellow gas; a dried treatment has a 100% chance of doing so if exposed to flame. The gas causes creatures beginning or entering the cloud to spend their turn retching and reeling. A DC 10 Constitution save negates or ends; a successful save grants immunity for 24 hours. A cloud disperses after 10 minutes (1 minute, dried).
Underdark terrain effects
Gravity near a mass of fey lodestone can behave erratically. Candle flames flicker or turn to balls of light, hair or clothing whips in unseen winds, and eventually even doughty knights may find themselves falling into a wall or ceiling. Weak lodenfloor, or -walls, or even -ceilings merely imposes disadvantage on ranged attacks that pass within ten feet of the surface and allow tiny creatures to stand on it as though it were the ground, regardless of orientation. Strong lodenfloor or other surfaces actually attract all creatures that pass within 60 feet of the surface. Depending on construction, this might result in a sixty foot fall upwards.