Definition
Difficult Terrain is a environmental modifier that negatively impacts a character's ability to move into and exit squares affected by the modifier, increasing their movement cost.
Effects
Difficult Terrain doubles the movement cost to enter or exit a square affected by it. So it would take 10ft of a character's speed to move 5ft into the square, and then another 10ft of speed to move into other Difficult Terrain squares and exit one. Effectively, it takes 20ft of speed to enter then immediately exit a Difficult terrain square. Dashing may be disabled or suppressed while in areas of Difficult Terrain, otherwise it might cause the Dashing character to have to roll a Saving Throw to avoid tripping up or running into something unpleasant. Obfuscating Terrain Obfuscating Terrain is a more severe form of Difficult Terrain that increases movement costs by x3.0 or more. Greater Obfuscating Terrain As Obfuscating, except instead of costing a multiple amount of movement, a creature must spend an entire Action frame to enter, and another to exit each affected square. Moving from one G.O.T. square to another effectively takes three action frames worth of time to move. This is as difficult to move through as possible before it simply becomes Impassible. Impassible Terrain This level of Difficult Terrain is a technicality only for situations where Terrain is so 'difficult' that it can't be passed into or out of by a character. It could be as mundane as a literal wall in the way you can't walk through, or having been slowed by effect in Obfuscating Terrain as to make movement impossible.
Triggers
Difficult Terrain can be the result of environmental obstacles such as uneven ground or debris making walking and running difficult. Some instances include but aren't limited to: ♦ Bodies of liquid that are higher than a humanoid character's shins or knees may be slowed due to pushing against the volume of liquid. ♦ Mud can make characters sink and stick into it's volume to slow them down. ♦ Semi-solid obstacles such as bushes and crowds of people can also count as one has to push through cracks in the density to get through and sometimes force isn't enough to do so. ♦ Narrow pathways and enclosed spaces can become Difficult Terrain if the space is narrower than the character's own natural Space. This can elevate to Obfuscating terrain if the character has to crawl into or squeeze into small spaces, and Impassible if the space is designed to fit a creature less than 1/4th the creature's natural size. Characters with Lean builds can gain bonuses to squeezing in small spaces whereas bulkier ones may suffer worse penalties. ♦ Reasons listed above can become or no longer become Difficult Terrain based on the character's Size. Extremely large creatures may have an easy time stepping over terrain that would hinder smaller ones, but at the same time have difficulty squeezing into or fitting inside places smaller ones have no trouble doing so.