Enormous spiders haunt the underground regions of the world, sometimes leaving sticky webs behind long after they themselves are gone.

    Effect: The webs of giant spiders are difficult terrain. A creature that enters a spiderweb must make an Athletics check or an Acrobatics check or become immobilized. Trapped creatures can use the escape action to free themselves from the web. A spiderweb square is lightly obscured.
    Usage: Don’t use spiderwebs only in encounters with giant spiders—they can also be interesting terrain in fights against either mobile monsters or monsters with push and slide effects. An ogre might try to push the party rogue into a spiderweb to make him stand still.

Published in Dungeon Master's Kit, page(s) 211, Dungeon Master's Guide, page(s) 69.