Homebrew conditions and hazards, and changes to official conditions, are marked with the ᴮᴹ tag.

BURNING [HAZARD] ᴮᴹ 

Ongoing Damage. A burning creature or object takes Fire damage at the start of each of its turns. For verisimilitude, the damage scales with the creature’s level or Challenge Rating: 1d4 (level or CR 1–4), 1d6 (5–10), 2d6 (11–16), or 3d6 (17+).
Self-Extinguish. As an action, you can extinguish the flames on yourself by falling Prone and rolling on the ground.
Environmental Extinguish. The flames go out if the burning creature or object is douzed, submerged, or deprived of oxygen.
Aid. You or another creature can use the Utilize action to empty a full waterskin (4 pints) onto the flames. A waterskin is considered full if it has not been used since your last Short Rest or if you specifically refilled it.

BURIED [HAZARD] ᴮᴹ 

While you are Buried, you experience the following effects.
Sight and Mobility Affected. You have the Blinded, Prone, and Restrained conditions.
Possible Suffocation. You must make a DC 13 Constitution saving throw. On a successful save, you have a lungful of air and can hold your breath for a number of minutes equal to 1 + your Constitution modifier (minimum 30 seconds) before suffocation begins. On a failed save, you can only hold your breath a number of rounds equal to your Constitution modifier (minimum of 1 round). When you run out of breath, you gain 1 Exhaustion level at the end of each of your turns. If you reach 6 Exhaustion levels, you fall to 0 Hit Points and start making Death Saving Throws (instead of dying outright).
Escape. As an action, you can make a Strength (Athletics) check to attempt to escape. A creature that isn't Buried can also use their action to make the check or assist you with the Help action. The DC equals 15 or the DC of the spell or effect that caused the condition. If you are freed and can breathe again, you remove all levels of Exhaustion you gained from suffocating, unless a spell or effect tells you otherwise. 

CORRODING [HAZARD] ᴮᴹ 

If a spell or effect specifies ongoing acid damage or a corroding hazard, that description takes precedence over this hazard.

Simple Contact. A creature that touches acid takes 5 (2d4) Acid damage.
Ingestion. A creature that ingests acid takes 10 (4d4) Acid damage and 10 (4d4) ongoing Acid damage at the start of its next 3 turns, or half the initial damage and no ongoing damage on a successful DC 13 Constitution saving throw.
Moving Into or Ending Turn. A creature that moves into an acid-filled area or ends its turn there takes 10 (4d4) Acid damage.
Submersion. A creature entirely submerged in acid takes 22 (4d10) Acid damage at the start of each of its turns and 11 (2d10) ongoing Acid damage for 3 turns after leaving the acid.
Ending Ongoing Damage. A creature can use an action to wipe away the acid. Magical healing also ends any ongoing damage.
Corrosive Scarring. A creature that takes more than 50 Acid damage from this hazard has its Charisma score* reduced by 2. If the reduction is not removed within 48 hours, it becomes permanent. The reduction can be ended early if the creature receives a Greater Restoration spell, a Potion of Superior Healing, or a 5th-level or higher healing spell such as Cure Wounds using a level 5+ slot. An Alter Self spell, or similar magic, can remove the acid scars and negate the Charisma score reduction for the duration.
Metal Objects. At initiative count 20 (losing ties), each nonmagical metal weapon submerged in acid takes a permanent −1 penalty to damage rolls, and each nonmagical metal armor or shield takes a permanent −1 penalty to AC. A weapon reduced to −5 damage, or armor/shield reduced to AC 10/+0, is destroyed.

* And Allure proficiency drops one level (Expertise → Proficient → Half Proficient → Not Proficient)

CROWDED [HAZARD] ᴮᴹ 

While in a Crowded urban environment, you experience the following effects:

Difficult Terrain. Your movement speed is halved as you navigate through throngs of creatures.
Disrupted Perception. You have Disadvantage on Wisdom (Perception) checks to perceive anything inside or outside the crowd.
Collateral Risk. When you make a ranged attack while Crowded and you roll a 1, 2, or 3 on your attack roll, your attack strikes a member of the crowd. Large area spells or effects such as Fireball requires a successful DC 10 Intelligence (Arcana) check to avoid hitting noncombatants.  
Frightened Commoners. Most commoners become Frightened if combat is instigated in the crowd. A Frightened creature takes the Dash action and moves away from danger by the safest route on each of its turns unless there is nowhere to move, in which case the creature attempts to Hide.
Mob Wrath. If you accidentally kill a crowd member and you are surrounded by creatures that are not easily Frightened (DM fiat), you must succeed on a Charisma (Deception, Intimidation, or Persuasion) check (DC 13 + 2 per additional incident) to calm the crowd. On a failure, the crowd transforms into a Hostile commoner mob that attacks until you are reduced to 0 Hit Points or the mob is Bloodied.

DEMON ICHOR [HAZARD]

Demon ichor is what remains behind after a demon dies. It’s a reduction of demonic blood, viscera, and bodily fluids with the consistency and odor of bile.
Flesh Warping. When a creature comes in direct contact with demon ichor, it must make a DC 10 Constitution saving throw. Fiends, oozes, plants, and undead automatically succeed on the saving throw. On a failed save, the creature is magically warped by the ichor, as determined by rolling on the Flesh Warping table. A spell that removes a curse ends all ichor-related warping effects on a creature.
Ichor-Coated Weapons. A creature that takes damage from a weapon coated with demon ichor suffers a flesh-warping transformation if it fails its saving throw. After hitting its target, the weapon is no longer coated with demon ichor.

See Bottled Demon Ichor magic item for the complete Flesh Warping table (Descent to Avernus).

MASSIVE DAMAGE [HAZARD]

When you take damage from a single source equal to or greater than half your Hit Point Maximum, you must succeed on a DC 15 Constitution saving throw or suffer a random effect determined by a roll on the System Shock table.

System Shock

d10	Effect
1	You drop to 0 Hit Points and start dying (Death saving throws)
2–3	You drop to 0 Hit Points but you are stable.
4–5	You are Stunned until the end of your next turn.
6–7	You can’t take Reactions and have Disadvantage on d20 Tests until the end of your next turn.
8–10	You gain 1d3 Exhaustion levels.

SUFFOCATION [HAZARD]

A creature can hold its breath for a number of minutes equal to 1 plus its Constitution modifier (minimum of 30 seconds) before suffocation begins. When a creature runs out of breath or is choking, it gains 1 Exhaustion level at the end of each of its turns. When a creature can breathe again, it removes all levels of Exhaustion it gained from suffocating.

SWALLOWED [HAZARD] ᴮᴹ 

While you are Swallowed by a creature, you experience the following effects:
Blinded and Restrained. Unless the swallowing creature is Gargantuan or its stat block says otherwise, you are Blinded and Restrained. However, if you attack with a Light weapon that deals Piercing or Slashing damage, you are considered neither Blinded or Restrained for that attack.  
Total Cover. You have Total Cover against attacks and other effects originating outside the creature that swallowed you. You have Disadvantage on Wisdom (Perception) checks made to hear.
Digestive Damage. At the end of each of your turns, you take damage as specified in the swallowing creature's stat block, or 10 (3d6) Acid damage if the stat block doesn't specify it.
Escaping. The stat block provide rules for escaping the creature. If not, the following applies: if the creature takes 20 damage or more on a single turn from a creature inside it, or the first time it becomes Bloodied, it must make a DC 15 Constitution saving throw. On a failed save, it regurgitates all swallowed creatures, which fall Prone in a space within 10 feet of the creature. If the creature dies, a swallowed creature can escape from the corpse by using half their movement, exiting Prone.

UNDERWATER COMBAT ᴮᴹ 

A fight underwater follows these rules. See also Suffocation [hazard]. 
Impeded Weapons. When making a melee attack roll with a weapon underwater, a creature that lacks a Swim Speed has Disadvantage on the attack roll unless the weapon deals Piercing or Psychic damage. A ranged attack roll with a weapon underwater has Disadvantage against a target within normal range, and automatically misses a target beyond the weapon’s normal range.
Fire Resistance. Anything underwater has Resistance to Fire damage.
Limited Spellcasting. Whenever a creature casts a spell with a Verbal component underwater, it must succeed on a DC 13 Intelligence (Arcana) check. On a failure, the muffled words and unstable magic trigger a Wild Magic Surge. On a natural 1, the spell fails outright and the spell slot or limited use feature is wasted.
Air Restriction. A creature that cannot breathe water can cast only one spell with a verbal component before it must surface for air.
Lightning Radius. Spells or effects that deal Lightning damage spread outward in water. Replace the effect’s normal area with a sphere centered on the point of origin: 10-foot radius per spell level (maximum 60 feet). The sphere affects all creatures in range, ally or foe, unless the spellcaster uses a special feature such as Sculpt Spell or Careful Spell. 
Deep Water (100+ feet). After each hour of swimming in deep water, a creature that lacks a Swim Speed must succeed on a DC 13 Constitution saving throw or gain 1 Exhaustion level.
Sonic Damage. Creatures take double Thunder damage (Vulnerability) while in deep water, due to amplified soundwave pressure. A creature that can magically breathe water or for which deep water is a natural environment ignores this vulnerability.
Visibility Underwater. Visibility underwater depends on water clarity and the available light. Use the Underwater Encounter Distance table to determine the encounter distances underwater. No natural light reaches below 200 ft.

Underwater Encounter Distance

Visibility	Encounter Distance
Clear water, Bright Light	60 feet
Clear water, Dim Light	30 feet
Murky water or Darkness	10 feet

Notes: Utility