The World Is Trying to Kill You (And It Has Rules About It)
The environment chapter is one of those sections people skip until it matters. Then a character falls off a cliff, or the party spends three days in a desert, and suddenly everyone wishes they’d read it.
Falling
The Player’s Handbook keeps falling damage deliberately simple: 1d6 bludgeoning for every 10 feet, capped at 20d6. No acceleration, no physics. A linear formula that treats the first 10 feet and the last 10 feet of a 200-foot drop identically.
Xanathar’s Guide adds two meaningful wrinkles. First, in any situation involving extreme heights, a creature can descend up to 500 feet instantaneously. Combat rounds being roughly 6 seconds, this is effectively terminal velocity modeled as a flat ceiling rather than a curve. Second, flying creatures whose speed is reduced to 0 immediately begin to fall. Hold Person on a humanoid with natural flight is not a hypothetical. It is a plan.
Tasha’s Cauldron of Everything adds one more piece: falling into water. You still take damage. A belly flop from 100 feet is not a soft landing. You can use your reaction to make a DC 15 Athletics or Acrobatics check to enter feet or hands first, which halves the damage on a success. At terminal velocity, water behaves like concrete. The rules know this.
Suffocating
You can hold your breath for 1 plus your Constitution modifier minutes, minimum 30 seconds. Once you run out, you have a number of rounds equal to your Constitution modifier, minimum 1, before dropping to 0 hit points. During that window, you cannot be stabilized or regain hit points until you can breathe again. Cure Wounds does nothing for someone actively drowning.
Vision and Light
Lightly obscured conditions, dim light, patchy fog, moderate foliage, impose disadvantage on Perception checks that rely on sight. Heavily obscured means the blinded condition entirely. Attacks into darkness are a combat question covered separately, but spells requiring a visible target simply do not function in heavy obscurity. The one exception that holds up rules-as-written: targeting yourself. You always know where you are.
Darkvision bumps you one step up the visibility hierarchy within its range. Darkness becomes dim light. Dim light becomes bright light. Everything appears in shades of gray. This is not flavor text. Color-based puzzles, identifying a gang by their colors at night, distinguishing a ruby from a garnet in someone’s jewelry, none of that works under darkvision.
Truesight cuts through everything: magical darkness, invisibility, illusions, and shapechanging. It is rare by design. An illusionist or a transmuter who regularly faces a truesight creature has a very bad time.
Food, Water, and Exhaustion
You need 1 pound of food and 1 gallon of water per day. In hot environments, water requirements double to 2 gallons. If you drop below half rations on food, the days accumulate against a threshold of 3 plus your Constitution modifier before exhaustion sets in. With water, every day at half a gallon triggers a DC 15 Constitution saving throw. Below that, you automatically gain one level of exhaustion, and if you already have exhaustion, you gain two.
Exhaustion is a condition worth understanding before you need it. Each level stacks penalties, and the only way to remove the exhaustion gained from starvation or dehydration is actually eating and drinking. You cannot long rest your way out of it. You cannot cast it away.
For reference: Player’s Handbook Chapter 8 covers the environment rules. Xanathar’s Guide and Tasha’s Cauldron of Everything each add the optional falling rules discussed above.