Guardianship of an ancient artifact, a hidden underwater system of tunnels and caves, along a depopulated, forested coast -- a world where human beings are few and far between, and magic has returned, but not in a fantasy novel sort of way. More like a rediscovery of an ancient undgerground river system of magic, and I and other merfolk guard the waterways from the few above who might stumble across it, misunderstand it, and get into terrible trouble.
One cave, a dry one, contained entry to an endless tunnel system, where monstrous dogs eternally chased a shining bleb of oil. The dogs were DANGEROUS; even worse was a witch who haunted those caves, hoping to catch other magical folk and enslave them, if they didn't know how to protect themselves. Amy showed me how to regain my mermaid's tail by catching the bleb, hopping lightly up onto the rim before the dogs could catch her, and drenching herself with the oil. It was a sensual experience, becoming aware of my skin and where it met my tail, the interface between the surface of the water and the surface of myself. Back into the water, swimming fast, and down a different set of tunnels, to where the other merfolk meet, guardians of the tunnels, in a cave full of light. Brent has known all along that I was mer, and is amused at my reaction.
One day, in the cave of the dogs, the witch catches Brent and I while I'm dousing myself in the oil. I grab him and swim like mad to the room where the merfolk meet; she knows about it already, and starts coming through the door. There are only a couple of people there, who I've never seen before; I take an emergency vial of oil and change Brent, tell him to swim north, heading for unpopulated territory (I've dreamed of other aspects of this world before -- a post-apocalyptic world, like that in Ursula LeGuin's Always Coming Home, forested and empty and silent). I face the witch, and she's unable to catch me; I evade her, and those in the room are just regular people, who know nothing about the artifact. I swim north for two days, and catch up with Brent, having lunch with all of the merfolk -- they knew the witch was coming, and left the two ordinary people as cover, to convince her that there wasn't really a group guarding the necklace -- that it didn't exist! I laugh and regain my legs, climb out of the water to go join them. We're in Oregon now, and I like it here. The cave in which the restaurant is is full of sunlight and relief. The neclace settles over my heart, safe, and I fit myself like a puzzle piece in the crook of Brent's elbow, leaning against his side.