This encounter revolves around Gadabouts, a monster from Spelljammer. I don't have any love for Spelljammer as a setting (that's a post I plan to write some time soon), but for some reason the stats are freely available on D&D Beyond. Gadabouts are essentially organic space suits that grapple a creature and surround its head in a bubble of oxygen. As written, they're supposed to be helpful to the PCs - but this encounter flips that concept on its head, presenting the Gadabouts entirely as they're written in official sources but recontextualizing them to be a threat. I thought it was a really cool idea to explore alternative ways to use monsters that were clearly made with a specific use case in mind (see also: cloakers).
This encounter consists of two parts: a pit of spikes, and an adjacent room containing a lush grassy area with a small pond, around which three Gadabouts hydrate themselves. In the campaign I got this from, this was in a dungeon and the pond room was concealed by an illusory wall, but it could work with a more conventional passageway instead, or even in a wilderness setting.
The pit is 10 feet deep and 5 feet wide. The spikes are coated in aboleth mucus - which can render a creature only able to breathe water if it is ingested or enters the bloodstream. Creatures that land on the spikes take 1d4 piercing damage and must make a DC 16 Constitution saving throw. On a success, they shake off the effects of the poison. On a failure, they initially experience difficulty breathing.
After ten minutes, an affected creature will find they aren't able to breathe air at all any more, and start to experience the effects of suffocating if their heads aren't immersed in water (ie. they can survive for a number of rounds equal to their Constitution modifier, after which they drop to 0 HP, begin making death saves, and can't be healed or stabilized until they can breathe again). The save can be retried every hour.
This might seem bad enough, but complicating matters are the Gadabouts in the adjacent pond. When at rest, they do not move and appear as lush vines with a giant butterfly sitting on top of them. However, if they are startled or a creature falls into the pit, they will fly toward the nearest creature in the pit (ideally one affected by aboleth mucus) and attempt to grapple them. On a successful grapple, the Gadabout attaches to the back of the target and surrounds them with a bubble of oxygen. They will then attempt to carry the grappled creature off; the target will be unable to move under their own volition until the grapple is broken. This means that the rest of the party will need to chase after the Gadabout before their friend suffocates. If the Gadabout is attacked, the damage is split evenly between it and the grappled creature.
The Gadabouts do not harm the creatures they attach to and are just trying to be helpful. They aren't aware of the aboleth mucus coating the spikes in the pit. However, the fact that they produce a bubble of air can prove dangerous if anyone in the pit is affected by aboleth mucus! This way, you can take a creature intended to be helpful and make it dangerous to the PCs without needing to change anything about its statblock - and present the players with a fun moral dilemma. How can they separate their ally from the Gadabout without harming them? And is it right to try and kill something that thinks it's being helpful?
