Webs

From Dungeon Master's Guide, page 105.

Hazard

Giant spiders weave thick, sticky webs across passages and at the bottom of pits to snare prey. These web-filled areas are difficult terrain. Moreover, a creature entering a webbed area for the first time on a turn or starting its turn there must succeed on a DC 12 Dexterity saving throw or become restrained by the webs. A restrained creature can use its action to try to escape, doing so with a successful DC 12 Strength (Athletics) or Dexterity (Acrobatics) check.

Each 10-foot cube of giant webs has AC 10, 15 hit points, vulnerability to fire, and immunity to bludgeoning, piercing, and psychic damage.