This encounter works best if the PCs are a couple levels lower than the EL. An area-affecting spell like lightning bolt or fireball can make quick work of the ophidians on the causeway. If this happens and the PCs are having an easy time with the encounter, consider having more ophidians climb up from below.
When the PCs look into the room, read this text:
Note: The drop is so deep that even a dwarf's darkvision cannot reveal what would stop a fall, so DMs should mention this if a party member has such vision.
Have the players make Listen checks and Spot checks. A PC that succeeds at the Spot check is not surprised when the ophidians slither onto the causeway. Read the following text:
PCs who failed their Spot checks are surprised, but the ophidians make no immediate move to attack. Instead, the first one waves with his sword in a threatening and challenging manner for the PCs to approach. The ophidians are guarding their mistress, a bright naga in counsel with two clerics of Bane several rooms away.
The ophidians speak Common but they can't be reasoned with or tricked unless someone speaks to them in Yuan-ti and claims to be a member of that race. This gives them pause, and successful Bluff and Intimidate checks convince them to lead the PCs to the bright naga and the clerics. The cowed ophidians explain the workings of the causeway and its dangers, and they escort the PCs, some going ahead to alert the bright naga of the "yuan-ti" visitors and others moving ahead and behind the PCs. Should the PCs keep this ruse going, it confuses both the naga and the clerics of Bane, allowing the PCs the element of surprise.
If the PCs talk for too long or start spellcasting, the ophidians nearest the entrance move to attack and the other ophidians file forward, all of them being curiously cautious to step in the diagonals. This gives their movement across the causeway a serpentine appearance that could be mistaken for their natural inclination, but a DC 15 Sense Motive check reveals they're trying to avoid certain areas of the walkway. Stepping in those areas (marked with an X on the map) results in the activation of a trap. See "Water Rush Traps" below for more details.
The two clerics of Bane have come to the bright naga's lair to convince her to undertake the ritual to become a banelar naga (see page 72 of Serpent Kingdoms). If the naga converts to worship of Bane, they intend to use her and her ophidian minions to guard a secret temple to Bane being carved out of Undermountain. Even now, they have hired adventurers working to clean out an area of monsters so that they can construct defenses and repurpose that area of the dungeon. The two clerics and bright naga come running as soon as a water rush trap is set off. They arrive in 4 rounds and enter the combat.
Water Rush Traps: CR 1: mechanical; location trigger; automatic reset; Bull rush +6; Search DC 15; Disable Device DC 20.
Each square marked with an X on the map is the trigger for hundreds of gallons of water to come rushing out of a hole in the wall to spray across the causeway over the two adjacent squares to the east. Thus, as someone moves across the causeway to the west, water sprays through the squares behind that person with each step onto a trapped square. The trapped squares cause a rush of water only when first depressed. Therefore, any creature stepping on a trapped square causes a rush of water when it first enters the square, but the water then ceases until the weight is removed and then put on the square again. It takes 10 pounds of pressure to cause a rush of water.
A creature in a square through which a rush of water passes is subject to a bull rush attempt. The water pushes with a +6 bonus on the opposed Strength check. If the water wins, the PC is pushed back 5 feet. If the water wins by 5 or more, the PC is pushed 10 feet. A PC pushed off the causeway may make a DC 15 Reflex save to grab the edge. Failure results in a fall to the water below (see Falling or Diving Off the Causeway).
The ophidians, clerics of Bane, and bright naga all know how these traps work and what squares are safe to step on. They use the traps to their advantage, staying off the trapped squares until a foe steps forward into the spaces the water would spray through. Whether a PC moves in front of or beside his foe, the PCs' foe need only 5-foot adjust to a nearby square to set off the trap and send a water rush to push the PC off the causeway. Of course, a PC might tumble or simply move around an ophidian, a tactic they are at first unprepared for, but then they stand side-by-side to block the causeway and step on and off the trapped squares to use the water against the PCs.
Make sure to keep track of the PCs' movement on the causeway. They too can set off the traps, and if they are not careful, a PC might end up accidentally setting off a water rush trap so that it affects another PC.
2 Clerics of Bane: Use the statistics for the Cleric of Bane (35/60) from the Archfiends miniature set. If you don't have that miniature, use Table 4-14 in the Dungeon Master's Guide to design two 5th-level human clerics.
Adventure Hooks and Tie-Ins
Need more information? Check the Return to Undermountain: An Introduction for more details.
About the Author
Once editor-in-chief of Dragon Magazine and now a game designer at Wizards of the Coast, Matthew Sernett wrote in a Dragon editorial that there's nothing in D&D he likes better than when the adventurers flee through the dungeon, running pell-mell through traps and past monsters because what chases them is worse. When he wrote that, Matthew was thinking about Undermountain.
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