Difficult Terrain
If a space is Difficult Terrain, every foot of movement in that space costs 1 extra foot. For example, moving 5 feet through Difficult Terrain costs 10 feet of movement. Difficult Terrain isn’t cumulative; either a space is Difficult Terrain or it isn’t.
A space is Difficult Terrain if the space contains any of the following or something similar:
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A creature that isn’t Tiny or your ally
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Furniture that is sized for creatures of your size or larger
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Heavy snow, ice, rubble, or undergrowth
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Liquid that’s between shin- and waist-deep
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A narrow opening sized for a creature one size smaller than you
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A slope of 20 degrees or more
Source · SRD 5.2.1 · page 181