The Environment

Not all dangers while adventuring are from enemies, monsters, and beasts. Whether urban or rural, the very environment a hero must traverse in order to complete their quest (or the battlefield they find themselves on) can present a multitude of hazards. Be sure to pay attention to the environment around your character as they move through it, and be wary of any hidden threats — or anything you can leverage to get the upper hand in a fight.

Falling

When a creature falls, it takes 1d6 bludgeoning damage for every 10 feet it falls (maximum 20d6) and lands prone. A creature that falls into water takes half damage, or no damage if it dives with a successful Athletics check (DC equal to the distance it falls divided by 5). 

Suffocating

Drowning, strangulation, smothering, strange alternate planes of reality with no air — many situations can lead to suffocation. A creature can hold its breath for a number of minutes equal to 1 plus its Constitution modifier (minimum of 30 seconds). Once a creature is out of breath, it begins suffocating and can survive for a number of rounds equal to its Constitution modifier (minimum 1 round).

At the start of its next turn, the suffocating creature drops to 0 hit points and is dying. It cannot regain hit points or be stabilized until it can breathe again.

For example, during a storm at sea, a creature with a Constitution of 16 is dragged beneath the ocean by debris tangled around its legs. It can hold its breath for 4 minutes, at which time it has 3 rounds before it drops to 0 hit points.

Underwater

A creature that cannot breathe water begins to suffocate underwater once it cannot hold its breath. In addition, creatures without swim speeds have disadvantage on attacks made using any weapons other than a dagger, dueling dagger, javelin, shortsword, spear, or trident. Ranged weapon attacks automatically miss beyond their normal range underwater, and bludgeoning and fire damage are halved. A creature that takes damage while holding its breath underwater must succeed on a concentration check or immediately begin suffocating as if its breath had run out.

Vision and Light

Most creatures rely on sight for even the most mundane, everyday tasks. Adventurers are no different, and the amount of light in a given area  impacts their ability to perform both in and out of combat. Other environmental features, like fog or heavy rain, may obscure a creature’s vision and cause difficulty.

A lightly obscured area is one that creates a minimal, but not insignificant, amount of visual impairment. A moderately wooded area, lingering smoke from a sacked town, dim lighting, and the like can all cause an area to be lightly obscured. When within this area, creatures have disadvantage on Perception checks that rely on sight.

An area that is heavily obscured may be a shoreline cloaked in dense fog, a jungle full of thick foliage and hanging vines, or darkness (magical or otherwise), that obstructs vision completely. A creature in this area is considered blind.

Three types of lighting exist within an area: bright light, dim light, and darkness.

Bright light is the most common. Even on a cloudy day the sun provides bright light, as do torches, bonfires, magical lights, and other similar sources (though often within a limited radius). Most creatures suffer no penalties from being in a brightly lit area.

Dim light creates a lightly obscured area. Dim light is typically found on the outer reaches of artificial light sources, such as the depths of a room lit only by candles or the outer ring of a bonfire. Dim light can also be found during dawn or twilight, or beneath a full moon.

Darkness creates a heavily obscured area. Creatures venturing out at night, delving underground, or traveling into a windowless dungeon without any alternate sources of light will find themselves at a distinct disadvantage.

Blindsight

Not everything relies on vision to sense the world. A creature with blindsight is not affected by darkness or other heavily or lightly obscured areas, and can see through invisibility, within a certain radius. Creatures adapted to the darkness (like bats and moles) or creatures without eyes (like purple worms) have blindsight.

Figments. Creatures that do not rely on visual sight, noted as having blindsight (blind beyond this range), are immune to visual illusions (such as those created by minor illusion).

Darkvision

Many creatures within the realms of medieval fantasy are gifted with sensitive sight. Darkvision allows a creature to see within darkness as if it were dim light, and dim light as if it were bright light, though the creature cannot perceive colors while in darkness, only shades of gray.

Truesight

Some especially powerful creatures, like particularly old dragons, have truesight. This allows a creature to see in normal or magical darkness, perceive invisible creatures and objects, automatically detect illusions and succeed on its saving throw against them, and perceive the original form of a shapechanger or creature that is transformed by magic. A creature with truesight can also see into the Ethereal Plane.

Food and Water

Creatures need to eat and drink in order to survive, and often in the wilderness, the opportunity to find fresh food and water can be limited.

A creature that takes a long rest must consume food and water or it suffers a level of fatigue (see the Supply rules).