In tonight's swamp hexcrawl game, the party planned to take a days long trek out to some submerged ruins, hoping to beat the villains there. To prepare, the druid spent some hard-won cash on components for a
continual flame spell and cast it, using up a spell slot he wouldn't likely get back before they delved the underwater structure. (Resting works differently here.)
One hex outside of town, I rolled up a random encounter: 8 Candlethieves. These are the spirits of children who got lost in the swamp and need light to find their way home. On their keel boat in the perpetual darkness of the swamp, the PCs had with them their
continual flame on the aarakocra druid's amulet and a bullseye lantern. After communicating with them briefly, the creepy little spirits moved in to steal their lights.
Action: The candlethief targets an object that sheds light within 30 feet of it and moves it up to 30 feet in any direction. The object can weigh no more than 15 pounds. If the target is an object being held, the chandlethief makes a Charisma check contested by the Strength check of the creature holding the object. If the chandlethief wins the contest, it can move the target as normal.
Though no hit points were actually at stake - the candlethieves have no damaging attacks - what followed was the most tense challenge of the night. The party is on a clock to make it to the submerged ruins, so turning around back to town will cost them. They are low on gold (and gold also equals XP!). They have no more components to recast
continual flame and, even if they did, that's another 2nd-level slot they can little afford to spend ahead of what might be a big confrontation with the villains.
What followed was a harrowing situation in which the PCs desperately tried to stop the candlethieves from getting the druid's amulet. They got it once, but some focused fire took out the spirit and the amulet fell into the drink. The aarakocra nosedived into the water as the candlethieves converged on him. Another candlethief stole the bullseye lantern off the ship and flew away. The druid cast
moonbeam (another precious spell slot gone!) and got some of them to walk into the light, but only narrowly kept his amulet before the rest of the party took out the spirits.
All that to say, giving the monsters something else to do other than whittle hit points can, in context, really make for some exciting challenges.