DOWN IN THE DEEPLAKE DEPTHS
Something sinister stirs in the un-plumbed fathoms of the Deeplake, and it's slowly killing the nearby town...
The Deeplake is an unusual body of water in that is more akin to a cistern of incredible proportions than an actual lake. The lake "bed" plunges straight down in smooth vertical walls, and the actual bottom is far out of sight.
The Deeplake is normally populated with a healthy variety of aquatic life, a surprising percentage of which can also be found swimming in the seas not far to the north and west. Scholars suggest that a heretofore-undiscovered passage connects the Deeplake with that larger, saltier body of water, but none have been able to prove it.
Still, the incredible diversity of life in the Deeplake has created a small cottage industry of underwater tourism. Well-to-do folk from Crestaria and Esse pay good money to be taken down in "seeing-bells", contraptions of thick steel and thicker glass deployed from the side of a sturdy boat that allow them to view the incredible watery depths in relative safety and comfort.
The more expensive and ornate seeing-bell operators boast expansive and completely dry viewing chambers illuminated with magical globes and filled with elemental air to allow deeper and more spectacular dives. The more economical the seeing-bell, the likelier chance of wet feet, and the cheapest of operations are riddled the stretch marks of metal crushed under massive pressure and hasty repairs.
THE DYING LAKE
In recent days, the Deeplake has undergone several disturbing developments. The waters have taken on a reddish cast, and anyone drinking of it complains of an increasingly metallic taste. The fish have almost completely disappeared - what little trace of them that remains is a few floating entrails. Seeing-bell operators report dark shapes surging up from the deep, and the larger bells and boats have been attacked by monsters that defy description.
The town elders are rightfully worried about the primary source of their livelihood, fresh water, and food, and so post a generous bounty for any person or persons who can go down into the lake and resolve the mystery.
DIVE, DIVE, DIVE
The most readily available method of exploring the Deeplake is through the use of the seeing-bells. Bells of all shape and size are open for business, although their operators are understandably nervous about deliberately searching for aquatic nightmare-beasts.
Seeing-bells can operate safely at different depths depending on the quality of their construction. The price of a trip, per person, in a seeing-bell, directly relates to the quality of the bell.
The cheapest seeing-bells cost 10 gp per person, and can safely operate at a depth of 10 feet. Each additional 10 gp per person spent on a seeing-bell trip increases the safe operating depth by an additional 10 feet, up to a maximum of 120 gp per person and a safe operating depth of 120 feet.
Every 10 feet a seeing-bell exceeds its maximum safe operating depth, there is a cumulative 5% chance that the bell catastrophically implodes from the pressure. Creatures inside the bell when it implodes must succeed a Dexterity saving throw with a DC equal to 8 + the number of 10-feet increments the seeing-bell has exceeded its maximum safe operating depth. Creatures that fail the saving throw take 5 (1d10) points of bludgeoning damage per 10 feet of depth and are restrained. When a creature is restrained in this way, it immediately begin suffocating. Creatures that succeed the saving throw take half as much damage and are not restrained. A creature can free itself from the restrained condition by using its action to succeed on a Strength (Athletics) or Dexterity (Acrobatics) check, with a DC equal to the DC of the original saving throw.
In addition to the danger of implosion, creatures in seeing-bells that dive below their maximum safe operating depth must also contend with air pressure. For every 20 feet a seeing-bell descends past its maximum safe operating depth, its inhabitants must succeed a Constitution saving throw with a DC equal to 10 plus 1 per 20 feet the seeing-bell has exceeded its maximum safe operating depth. Creatures that fail the saving throw gain one level of exhaustion. Creatures that do not need to breathe or are immune to poison do not need to make this saving throw.
A seeing-bell that is returning to the surface must stop every 20 feet for at least 3 minutes. Creatures in a seeing-bell that do not make this stop must make a DC 12 Constitution saving throw. This DC increases by 1 for every 20 feet the seeing-bell has ascended. On its first failed saving throw, a creature takes 3 (1d6) damage. On its second failed saving throw, a creature takes 5 (1d10) damage and is poisoned. On its third failed saving throw and every failed saving throw thereafter, a creature takes 7 (2d6) damage and is also paralyzed.
All seeing-bells come equipped with a number of suits that allow the wearer to operate outside the bell without having to hold their breath. A suit reduces its wearer's speed to 10 feet and can only travel 30 feet from the seeing-bell. While wearing a suit, creatures have disadvantage on Dexterity checks and saving throws.
WHAT LURKS BENEATH
This set-up is perfect for adventurers at any stage in their career. Lower-level parties will be vastly more dependent on the seeing-bells than their higher-level counterparts, so you could consider playing up both the luxury and safety (hah!) of the seeing-bells and/or the danger of free-swimming in the exposed waters of the Deeplake. Besides, who doesn't want to see the party barbarian in a diving suit?
For low-level parties, an appropriate encounter would be a colony of aquatically-adapted myconids. Given the benign nature of myconids, this is also the version of this encounter that is most likely to end peaceably. This encounter is likely to happen at a depth of about 20-40 feet.
For a more middle-range adventure, have the party discover a colony of merrow or sahuagin. A great way to soften up the players is to have them encounter a few sharks on the way down that seem hungrily interested in the cables that connect the seeing-bell to the ship above... This encounter is likely to happen at a depth of about 40-60 feet.
For a higher-end adventure, the party can encounter an aboleth and its minions. Chuuls work as excellent foot-soldiers and as threats to chase adventurers as they ascend and descend through the depths. Merrow and sahuagin can also provide unpleasant surprises when they burst up from the open bottom of the seeing-bells. This encounter is likely to happen at a depth of about 60-100 feet.
For truly epic adventurers, challenge them with a kraken at the bottom of the Deeplake. Consider having some citizens of the town (and specifically, some of the seeing-bell operators) already be thralls of this legendary creature, adding a lovely Lovecraftian atmosphere to the proceedings. The kraken should only be encountered after several harrowing attempts to cut the characters off from the surface, whether it be destroying the life-preserving and air-supplying cable to the anchoring ship or by frontal assaults into the seeing-bell itself. The kraken might also summon a terrible storm to batter the town, causing even the sturdiest of ships risk being dashed against the rocks or crushed by towering waves. This encounter should only happen at a depth of 100-120 feet, and can easily happen as far down as you would like. At a depth of 200 feet, even the most resilient seeing-bells is likely to crack under the pressure.