Falling [Hazard]

Source: D&D 5.2 SRD Rules Glossary

A creature that falls takes 1d6 Bludgeoning damage at the end of the fall for every 10 feet it fell, to a maximum of 20d6. When the creature lands, it has the Prone condition unless it avoids taking any damage from the fall.

A creature that falls into water or another liquid can use its Reaction to make a DC 15 Strength (Athletics) or Dexterity (Acrobatics) check to hit the surface head or feet first. On a successful check, any damage resulting from the fall is halved.