The heroes come upon a pool of fresh water in the forest or a strange dungeon. The pool seems normal but radiates strong magic. The water is normal (and non-magical, if removed from the pool), but anything completely submerged in it is turned to stone. The heroes can even see a few stone “statues” at the bottom (whatever creatures are common to the area.)

One of the “statues” visible thru the clear water is a nude man. A little investigation will turn up his clothing, neatly folded on a rock nearby but badly deteriorated by time. This man, they can tell from the clothing, is Ardonis Carumar, the person the heroes have been sent to find (an absent-minded wizard or nobleman, perhaps). Ardonis drank his fill and decided to take a swim but, once completely submerged, was turned to stone.

A submerged creature is allowed a saving throw, but inanimate objects are not. Success means it’s merely affected as if by a Slow spell. If a petrified creature is not magically returned to flesh before a year and a day, it remains stone forever.

The petrified man weighs 580 pounds. The heroes are free to try to figure out ways of hauling Ardonis up using a rope, net, or other item. A grappling hook attached to the rope would be completely submerged and turn to stone, but that might not affect its capability, just its weight. A rope or net would not turn to stone, as long as one end is still out of the water. A spell like Levitate might work, depending on the strength (the standard spell in 5e is limited to 500 pounds).

If returned to flesh, Ardonis will be very grateful and remembers perfectly well how he got petrified.

Variations

You might make the petrified man lighter (different stone types have different densities), so a Levitate spell would work.

You might say that only living creatures get turned to stone, so inanimate objects aren’t affected. But this will make the pool less fun to play with, since the heroes might come up with any number of odd things to do at such a pool.

You might use the year-and-a-day rule to create a ticking clock, so the heroes can’t just go back to town and get some laborers, a gantry, and a block and tackle set. The heroes didn’t learn about the missing man opportunity until nearly a year had passed, because the Carumar family announced the offer of a reward when thinking about the upcoming one year anniversary of his disappearance. Perhaps they could spare one day to rig their own makeshift gantry but not the two days it would take to go back to town and bring workers back. This would mean the heroes would need to have some way to turn Ardonis back to flesh almost immediately, so they’ll need a Stone to Flesh scroll on hand from another adventure or some other access to the spell.

Even knowing about the year-and-a-day rule might be a lore check that they heroes might have to make. There’s no reason the family would know that Ardonis had been petrified….

…Unless they had used some sort of augury magic to do so, and now they know they must act fast, in which case the heroes might be sent on the adventure knowing full well they’d need to get Ardonis returned to flesh. They might have been given a Stone to Flesh scroll–which the heroes might use on one of their own, if something unfortunate happened.