Snake Pit

A rickety wooden walkway threads around the walls of a deep pit. Foul odors waft up from below. Somewhere in the gloom beneath you, thousands of snakes writhe.

Despite appearances, the rickety wooden walkway is safe. The yuan-ti sometimes hurl disobedient slaves into the pit to be devoured by the serpents. The pit is 50 feet deep and filled with poisonous snakes. When a creature falls or sets foot in the pit, a swarm of poisonous snakes forms around it and attacks. A virtually inexhaustible number of swarms can be created in this fashion. A swarm that has nothing to attack ceases to exist as its constituents slither apart.
To climb the wet cavern walls, a character must succeed on a DC 15 Strength (Athletics) check.