Delver’s Guide: Tidal Well

A massive, round plug of reinforced timber hangs tipped forward into the entrance of this damp chamber, leaving enough room to squeeze past. Within, a 300-foot-long stretch of algae-slicked, pockmarked stone lies before you. Pools of standing water cover the floor, and a steady sheen cascades down portions of the walls from unseen sources. Shells crunch and anemones squish underfoot. Smaller grottoes lead off the main chamber. At the other end, an identical wooden plug tips toward you… and the water’s rising.

If you’re going adventuring, you best be prepared. Know what you might be getting yourself into and be ready for anything. It’s important to be mindful of your environment. Use it to turn any situation to your advantage.

Tidal Well

Perhaps not necessarily intuitive, water is a common feature when delving underground—both help and hindrance and not to be casually dismissed. The chamber is riddled with tiny fissures that channel seawater, whether from aboveground or from an underground sea, and regularly fills with water from tidal action.

Tidal Lock. This particular chamber has artificial plugs at each end that seal the chamber when it is filled with water. (Enterprising locals may have been trying to limit the effects of flooding, which can devastate confined underground settlements. Alternatively, the locking mechanism could be less intentional, perhaps owing to more natural plugs of stone and debris.)

The chamber remains either filled with water or fully drained for 1d8 + 6 rounds and then actively drains or fills with the tide for 1d4 rounds and then repeats the cycle. It can be first encountered at any point in this cycle, but a sample cycle follows:

• The chamber is empty of water, excepting the pools of standing water everywhere, for 1d8 + 6 rounds.
• Then the tide starts to rise for 1d4 rounds.
• Once full, the plugs are sealed, and the water remains for 1d8 + 6 rounds, completely submerging all within.
• Then the tide recedes, and for 1d4 rounds, the chamber drains.
• Repeat cycle.

When the chamber is empty, all surfaces are slick with algae and littered with broken shells and barnacles, making for challenging walking and climbing: treat as difficult terrain, and you cannot take the Dash action.

During the rounds when the chamber is actively filling or draining, the rapidly flowing water is also considered difficult terrain, and you must succeed a DC 10 Strength (Athletics) or Dexterity (Acrobatics) check to be able to move at all; if you fail this check by 5 or more, you are knocked prone and take 1d6 bludgeoning damage. (If you have a swimming speed, then these checks are made at advantage.) During these rounds, you must make a DC 14 Strength (Athletics) or Dexterity (Acrobatics) to enter or exit past the plugs; only Large or smaller creatures can fit through at all.

When the chamber is full of water, the plugs seal and are immovable from either side due to water pressure, giving way only to magic or extreme violence. The chamber is completely submerged, and there is no air to be had, but the water is no longer considered difficult terrain.

Additional Hazards

These tidal wells are rife with life, from the ubiquitous algae to various crustaceans and mollusks, not to mention the underground scavengers that have adapted to the watery intrusion. But there are some less common occurrences that stand out:

• Coral Drake. The tide often brings eggs and small sea spawn through the fissures—most never fully mature due to the elements and predation or simply pass through to whatever body of water lies beyond, but occasionally, a variety of sea life beyond the usual finds purchase here. And so a coral drake (see Tome of Beasts), trapped in the well from its earliest days, has adapted to the well by developing the ability to breathe in both air and water (replace the creature’s Water Breathing trait with Amphibious). However, it still relies on the water, so it is unable to wander far from the well, and it has not yet found any alternative. It might be open to negotiating with explorers offering a solution to its predicament, but it has found little reason to trust others down here.
• Deep Gnomes. A group of ingenious deep gnomes are running experiments to try to magically capture the energy of the tide within the well, to combine it with their more ingrained affinity for earth magic. They would have much knowledge to offer anyone able to help them crack these secrets.
• Garroter Crabs. The slight background clattering erupts in volume as garroter crabs (see Tome of Beasts) swarm from shadowed crevices. The tidal pools prove a rich feeding ground for these scavengers.
• Yann-An-Oed. The stranded yann-an-oed (see Creature Codex) has spent too many years lairing in the rich abundance of this chamber and has grown too large to leave.

Notes on Lethality

The goal here, and with all of the rooms in Delver's Guide, is not to create death traps. (Save that for another time!) These rooms are not intended primarily, as presented, to kill characters directly but rather to offer additional challenges and complication. But suffocating becomes a real threat here potentially. 

Suffocation. It would be good to review the suffocating rules before utilizing this room. Note that characters with a Constitution of 10 can hold their breath for 1 minute (or 10 rounds) and then get 1 "free" round of choking before having to roll death saving throws, so even the unluckiest of characters (rolling a 1 on a death saving throw) would have 13 rounds before dying, assuming no intervention from other characters or circumstances, so the current tidal action (1d8 + 6 rounds) is at the very edge of lethality. Unfortunately, anyone with a Constitution under 10 can only hold their breath for 30 seconds (5 rounds), so this would be very risky for them. It is recommended you consider the effect you're going for (and how lethal you want it) and the party at hand in order to tune the tidal action: as example, at 1d4 + 6 rounds, no one in a party with 10 as the lowest Constitution score would actually risk drowning but would still have a potentially memorable experience. 

Encounters. Also of note is that an added encounter with an aquatic/amphibious creature is going to play to that creature's strength, potentially increasing the intended CR of the encounter. To avoid this, consider encounters where the opponent is on the same footing with regard to not being aquatic/amphibious. Perhaps some pechs (see Warlock Grimoire 1) or ratfolk (see Tome of Beasts) got trapped along with the party: the former could hold their breath for a couple minutes but have no particular skill at swimming while the latter have a swim speed but only average ability at holding their breath. This ensures the challenge affects both sides of the conflict equally. 

Mitigation. There are many ways to mitigate potential suffocation, especially for lower-level parties where preventative and lifesaving magics and high constitutions may not be abundant. Consider that some large vacated gastropod shells are about, offering 1 extra round of breath to an enterprising character partaking of the air trapped within. Perhaps that one party member with a particularly high Constitution might suggest some quick mouth-to-mouth to share their air. Or maybe the last party to perish had a bag of holding among their remains: there's up to 10 minutes of air in those (depending on the number of creatures)!

The point here is to make sure it's only as lethal as you want.

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The Delver's Guide series explores the exciting world of dungeoneering by giving you interesting new locations to explore! Each is intended to give encounters a little something extra, designed to complicate encounters with more varied, dynamic, and exciting options, with hazards to avoid and features to utilize and surprises for the unwary. Are you ready for the delve? (by Scott Gable, Warlock Editor).