Quicksand

From Dungeon Master's Guide, page 110.

Wilderness hazard

A quicksand pit covers the ground in roughly a 10-foot square area and is usually 10 feet deep. When a creature enters the area, it sinks 1d4+1 feet into the quicksand and becomes restrained. At the start of each of the creature's turns, it sinks another 1d4 feet. As long as the creature isn't completely submerged in quicksand, it can escape by using its action and succeeding on a Strength check. The DC is 10 plus the number of feet the creature has sunk into the quicksand. A creature that is completely submerged in quicksand can't breathe.

A creature can pull another creature within its reach out of a quicksand pit by using its action and succeeding on a Strength check. The DC is 5 plus the number of feet the target creature has sunk into the quicksand.