plot element I don't have a place for... FROM A DREAM THIS TIME:
merfolk have a centuries-long cunning plan that they are superstitious about. they also have a tree at the sea floor that is linked with their plan. the tree has green-yellow semi-transparent teardrop-shaped leaves, and its trunk and branches are thick and covered in polygonal patterns that scatter light. many of its branches spontaneously stop growing before they grow leaves and branches of their own. the tree grows as the plan comes closer to completion, and when the tree is damaged there is a set-back.
the tree generates a dome of illumination and breathable air (think jar-jar's people's houses). a part of the plan is that a member of a non-aquatic sapient race has to live with the tree (maybe it just needs a sapient and merfolk can't live in air too long). when they need one they accept anyone they don't think is a saboteur, so mostly fugitives because it's not a fun job. because people (at least humans) can't keep from fiddling with things, this person needs to be either restrained while alone, or lack arms (this is not a part of the plan, it's a result of a committee trying to work out how to stop them hurting the tree). because the tree is so important, it is well-guarded. the person who lives in the dome gets entertainment (mostly books), and sustenance (mostly fish), from the guards. the person is told they will be executed if they hurt the tree, and that the tree needs someone to live with it, but nothing of the plan or the tree’s connection to the plan.
really, the function of this person is to be a scapegoat when something goes wrong: they're accused of hurting the tree (which always looks harmed because it aborts its own branches) and then executed and replaced. this is the only time they’re told about the connection. in the dream, the setback was a plague that killed three of the POV's friends.