Dungeon Vignette articles will describe a room or small area within a dungeon that can be easily dropped into another adventure or be used to spark ideas.
The characters will find their way down into the caverns and discover a small underground lake. The entire cavern is obscured, not by fog, but by smoke above the water's surface. The water is dark and very still. It is silent in the area and any noises echo.
It is not obvious where the smoke originates. There are actually two sources. Close to the lake center is a tiny island on which rests an open chest. Within the chest is an Eversmoking Bottle with the top off. That accounts for the heavily obscured 120' radius. The second source originates further back in the cavern on the far side of the lake.
The 120' radius area around the bottle is heavily obscured causing the blindness condition. The DM is encouraged to randomly determine whether each 10' area beyond the bottle's 120' radius is lightly or heavily obscured.
A fumarole has opened in the cavern wall, spewing toxic smoke into the area of the lake. The fumes require a DC15 Constitution saving throw every 10 minutes. Failure results in those affected accruing a level of exhaustion. Cumulative exhaustion may lead to character death.
There are two small rowboats tied to rocks along the lake shore. player characters can use these to paddle out into the lake.
The smoke makes it possible for the boats to get separated. The DM is encouraged to roll randomly for course changes (left 45 degrees, right 45 degrees, or directly ahead) when a boat is in a heavily obscured area. (perhaps use a d8 with 1-2 left, 3-6 ahead, 7-8 right) Lightly obscured areas allow for course correction.
There should also be an encounter once the characters reach the island. Some denizen of the lake with blindsight, aim for a medium challenging encounter, takes umbrage at trespassing. The encounter is made more difficult because of the obscuring smoke, the risk of possibly fighting in the water, and any levels of accumulated exhaustion.
Whatever McGuffin the party is seeking should also be on the island. Locate Object seems like a great spell to prepare for this situation. Make the trip worth the trouble.