Since the dawn of time, mermaids have been able to make pearls through magic. (Mermen create seashells, and these are what the mermaids wear.) These pearls form in the center of their seashell top, right over their heart, over and over throughout their lifetime, and hold a bit of the mermaid's essence (they literally put their heart and soul, part of themselves, into their pearls. Don't worry though, what they lose they eventually regain since the heart and soul are immortal.)
Since each pearl contains a piece of their self, mermaids give them to their loved ones as a sign of their bond; for spouses, this strengthens their marital bond, while for their children, it brings out their potential and allows them to feel their mother's love for them within their own hearts. This is even more touching when one considers that a pearl gives one power over its creator; if you have a mermaid's pearl, you have power over them.
However, now that Atlantis has removed its Barrier of Secrecy there's a problem: everyone wants their pearls. Human men seem to sense the significance of the pearls and desire one (or more, there are hoarders) for their own, and if that wasn't bad enough, they have an uncanny knack for finding them. Additionally, witches have realized that they can claim a mermaid's pearl, adding her magic to their own and getting a new slave as well. They (and other mages as well) also want to claim the pearls as ingredients for their potions or spells, for twisted experiments, and who knows what else. Sea lamia (yes, there are sea lamia, just as there are sea snakes) desire the pearls for use in subjugating the mermaids, which they are incredibly jealous of (humans like mermaids better, so they feel wronged and want revenge). Even crazy dragons want these pearls! (see ("https://worldbuilding.stackexchange.com/questions/193057/how-dragons-can-hoard-people") if you're curious as to why)
So now the question is: How to defend the mermaid's pearls?
- While the deep-sea location of Atlantis is a significant barrier, there are spells that negate even that advantage. (Magical submarines are tricky to make, but they are a thing.)
- A simple "force field" can be broken or bypassed, so that's not a solution. Something more restrictive, like the Age Line from Harry Potter and the Goblet of Fire would be preferred.
- Since no defense is invincible, and no defenders can repel would-be pearl thieves forever, a secretive location is best.
- Secrecy, as mentioned above, are your friend, as are complexity and redundancy. You're designing a magical security system here, and stakes are high if it fails.
- While magic exists, powerful magic is rare and powerful enchantments are even rarer. Setting up something like the Ark of the Covenant, that'll disintegrate anyone who's dumb enough to open it up, is not possible. Neither is impenetrable defenses or an impossible maze. Everything magical has to have weaknesses; the more "unfailable" you make it, the less likely it'll work. Simple but effective is the key here.